Support for multi-measurement benchmarks

This release adds support for Tachometer's multi-measurements per benchmark. Because it requires changing our comment format in non-backwards compatible way, this is a major release.

Any existing PRs that merge in this version with v1 comments may get comments that look a little weird. If you'd like to fix them, just delete the comment and re-run the jobs. They will recreate the comment with the new format.

List of changes:

• Support reporting multiple measurements from one report (#22)
• Let GitHub auto markup commit to get hover effect (#21)
• Remove GitHub check code and experimental use-check option