sort sub-benchmarks by name (fixes #179)

[mykmelez]

This sorts the sub-benchmarks by name when generating an HTML report, resolving #179 (assuming you agree that we should fix that! I know that this PR is putting the cart before the horse, as we haven't had any discussion in that issue yet, but I figured it would be a small change, and I wanted to familiarize myself with the codebase).

Note that CONTRIBUTING suggests to run cargo +nightly fmt, but that currently fails for me with:

thread 'main' panicked at 'not yet implemented', tools/rustfmt/src/expr.rs:346:47

Which is rust-lang/rustfmt#2743. However, I don't foresee any formatting issues with this small (two-line) change. And in any case rustfmt makes a bunch of other changes to the code before panicking, so it looks like the codebase hasn't been getting formatted lately (or at least not with recent versions of rustfmt!).

[coveralls]

Pull Request Test Coverage Report for Build 425

• 0 of 2 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.004%) to 12.105%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/html/mod.rs	0	2	0.0%

Totals
Change from base Build 424:	-0.004%
Covered Lines:	410
Relevant Lines:	3387

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💛 - Coveralls

[bheisler]

Thanks for the pull request!