refactor: Fix how indices are computed, add attrib indices

[fb55]

The previous approach had several short-comings; eg. special comments like </12> or CDATA in HTML would have misreported indices.

As a new feature, attributes will now have indices set appropriately.

[coveralls]

Pull Request Test Coverage Report for Build 1175005794

• 28 of 29 (96.55%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 98.488%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/fixtures/test-helper.ts	2	3	66.67%

Files with Coverage Reduction	New Missed Lines	%
src/Parser.ts	1	98.83%

Totals
Change from base Build 1174398383:	-0.2%
Covered Lines:	912
Relevant Lines:	926

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💛 - Coveralls