Adding findByteRange helper

[waaronking]

Per #98, this PR adds a findByteRange that properly parses the ByteRange field regardless of the spacing included inside.

The function returns a byteRangePlaceholder (if it exists), a byteRangeStrings array containing each byteRangeString in the pdf, and a byteRanges array of values for each byteRangeString. I've updated the PR to support multiple byteRanges per PDF.

[coveralls]

Pull Request Test Coverage Report for Build 329

• 14 of 14 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 325:	0.0%
Covered Lines:	303
Relevant Lines:	303

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💛 - Coveralls

[waaronking]

Per the requirements for this PR, only byteRangePlaceholder was required, but since the other data was easily accessible, I added byteRangeStrings and byteRanges that could be used to refactor/replace some of the logic in https://github.com/vbuch/node-signpdf/blob/develop/src/helpers/extractSignature.js at some point in the future.

[vbuch]

Great contribution!

[waaronking]

Thank you! And thanks for all the work you've put into this project!