Closed #2.

Added `yuno prune` to remove references to missing tests from history
and suite files. Small supporting tweaks to the loading abstractions.
Fixed #5.
Closed #6.
Closed #4.

The core.testing.Test class now strips leading ./ from paths. Changes
to util.multiline_fill allow the empty string as a Test object's path_to.
When sent to output templates, empty leading paths (for tests directly in
the repo root) are rendered as '[repo root]' instead.
Better ^C messages.
Readme updates for #6.
Improved check-range semantics and updated readme.

Check and phase ranges now do greedy, inclusive range matching by
default and don't have any special cases. Exact matches are exact;
ranges include every part of every check they touch unless specifically
restricted by a letter on the start or end point(s). Details in the
readme diff.

Falling back to regex-matching on the repo in more cases has reduced
performance somewhat, but in this specific case it may be possible to
optimize the difference out.
Performance improvements for common use cases.

Instead of loading all the tests and filtering, running phase/check
ranges now regex-matches path segments as it searches. Phase and
check together will be faster since the path is more specific, but
check alone should also see some gains.

Also fixed some lurking corner cases with parsing phase/check
combinations.

Related: #6.
Talked `certify` out of eating the run log.
More input checking for CLI commands.