Add cell type enum, date/sharedStrings helpers

[MaxDesiatov]

New stringValue and dateValue helpers on the Cell type are added, which allow getting corresponding string (including shared strings) and date values easier.

Resolves #71
Resolves #89

[codecov]

Codecov Report

Merging #102 into master will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
+ Coverage   98.12%   98.23%   +0.11%     
==========================================
  Files           8        8              
  Lines         267      284      +17     
==========================================
+ Hits          262      279      +17     
  Misses          5        5              

Impacted Files	Coverage Δ
Sources/CoreXLSX/Worksheet/CellQueries.swift	100.00% <100.00%> (ø)

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