Make bcf::IndexedReader::fetch functionality inline with htslib regions

[mbhall88]

Currently, an end position is required when fetching. This PR changes the fetch method so that end is now an Option<u64> and setting it to None will fetch from start until the last record of the contig.

I've also added

• Some clarifying docs about the fact that the start and end positions are both 0-based inclusive
• More test cases for fetch (including to test the changes)
• Doc example of how to use bcf::Record::format

[coveralls]

Pull Request Test Coverage Report for Build 1000005002

• 25 of 25 (100.0%) changed or added relevant lines in 1 file are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 93.314%

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Files with Coverage Reduction	New Missed Lines	%
src/bam/record.rs	7	74.3%

Totals
Change from base Build 837160575:	-0.02%
Covered Lines:	11375
Relevant Lines:	12190

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💛 - Coveralls

[brainstorm]

Makes sense and good accompanying docs and test, thanks for this!

[johanneskoester]

Awesome, thank you!