Add Table Title for Excel Output

[GoFroggyRun]

Currently TaxBrain Excel output files do not include Table title. As demonstrated below:

This PR adds a function addTit, within ext.buttons.excelHtml5, to insert title information to excel output (excel button only, CSV and copy buttons have different logic and thus will be fixed separately). Selector is used to extract proper title names.

This PR generates excel files with title information included as follow:

Could you review? @MattHJensen @brittainhard

[GoFroggyRun]

Commit a3c65cf and d6d6c11 allow table generated by Copy and CSV buttons to include titles.

Before the two commits, Copy button on TB would generate table that looks like:

    2017    2018    2019    2020    2021    2022    2023    2024    2025    2026
    Billions    Billions    Billions    Billions    Billions    Billions    Billions    Billions    Billions    Billions
Individual Income Tax Liability Change  0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Payroll Tax Liability Change    0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Combined Payroll and Individual Income Tax Liability Change 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Source: http://127.0.0.1:8000/taxbrain/40/        

or

    Tax Units with Tax Cut  Tax Units with Tax Increase Count   Average Tax Change  Total Tax Difference    Percent with Tax Increase   Percent with Tax Decrease   Share of Overall Change
    Thousands   Thousands   Thousands       Billions            
0-10%   0   0   17,779  0   0.0 0.0 0.0 0.0
10-20%  0   0   17,781  0   0.0 0.0 0.0 0.0
20-30%  0   0   17,780  0   0.0 0.0 0.0 0.0
30-40%  0   0   17,780  0   0.0 0.0 0.0 0.0
40-50%  0   0   17,781  0   0.0 0.0 0.0 0.0
50-60%  0   0   17,780  0   0.0 0.0 0.0 0.0
60-70%  0   0   17,780  0   0.0 0.0 0.0 0.0
70-80%  0   0   17,779  0   0.0 0.0 0.0 0.0
80-90%  0   0   17,781  0   0.0 0.0 0.0 0.0
90-100% 0   0   17,781  0   0.0 0.0 0.0 0.0
All 0   0   177,802 0   0.0 0   0   0.0
Source: http://127.0.0.1:8000/taxbrain/40/   

And right now, after the fix:

TOTAL LIABILITIES CHANGE BY CALENDAR YEAR (2017)
    2017    2018    2019    2020    2021    2022    2023    2024    2025    2026
    Billions    Billions    Billions    Billions    Billions    Billions    Billions    Billions    Billions    Billions
Individual Income Tax Liability Change  0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Payroll Tax Liability Change    0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Combined Payroll and Individual Income Tax Liability Change 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Source: http://127.0.0.1:8000/taxbrain/40/      

or

COMBINED PAYROLL AND INDIVIDUAL INCOME TAX: DIFFERENCE BETWEEN BASE AND USER PLANS BY EXPANDED INCOME BIN (2020)
    Tax Units with Tax Cut  Tax Units with Tax Increase Count   Average Tax Change  Total Tax Difference    Percent with Tax Increase   Percent with Tax Decrease   Share of Overall Change
    Thousands   Thousands   Thousands       Billions            
Less than 10    0   0   22,087  0   0.0 0.0 0.0 0.0
10-20   0   0   23,833  0   0.0 0.0 0.0 0.0
20-30   0   0   18,656  0   0.0 0.0 0.0 0.0
30-40   0   0   17,298  0   0.0 0.0 0.0 0.0
40-50   0   0   13,359  0   0.0 0.0 0.0 0.0
50-75   0   0   24,559  0   0.0 0.0 0.0 0.0
75-100  0   0   15,353  0   0.0 0.0 0.0 0.0
100-200 0   0   26,243  0   0.0 0.0 0.0 0.0
200-500 0   0   9,221   0   0.0 0.0 0.0 0.0
500-1000    0   0   1,357   0   0.0 0.0 0.0 0.0
1000+   0   0   710 0   0.0 0.0 0.0 0.0
All 0   0   177,802 0   0.0 0   0   0.0
Source: http://127.0.0.1:8000/taxbrain/40/          

Likewise, for CSV button, the generated data table will include title, as shown in the follow screenshots:

[brittainhard]

LGTM, +1 for merging

[brittainhard]

@GoFroggyRun can you handle these merge conflicts in this PR and the one in #564?

[GoFroggyRun]

@brittainhard , thanks for the reminder, this PR is good to go.