Add command-line argument "version" in test-runner

[amanbh]

Tests for multiple versions may be kept as subdirectories under the tests directory. This PR adds support to select a target version to test against by specifying the --version argument. The default value is the latest version.

This commit should be compatible with #3

[pradyunsg]

I don't think we should do this -- instead, the compliance test suite should only target a single version of the TOML spec, and get released alongside (possibly with bugfixes).

[eksortso]

I'm inclined to agree, @pradunsg. A tagged branch of the compliance project can be kept for tagged versions of TOML, and new developments can be matched with merged-in tests downstream. No need to maintain all versions at once.