Bash (globstar) style globbing

[hdgarrood]

Implement bash style filename globbing for data-files (well, bash after doing shopt -s globstar). Subsumes #1343 and #1344.

The plan:

• * can expand to a part of any file or directory but not descend into subdirectories
• ** is the same, except that it can match any number of levels of directories (including 0).
• {a,b,c} can expand into a, b, or c, where a, b, and c are globs themselves. So {foo,*bar} expands to "foo" or "abar" or "bbar"...
• "Literal" and glob patterns may be mixed to an unlimited extent in a single directory level. That is, foo/**/* is allowed, and so is foo/**/*.hs, and foo/**/Test*.hs, and foo/**/*{Test,Spec}*.hs.
• Backslash \ may be used to match any of the charactes {, }, *, \ literally. Eg \\ matches \.

I intend to push some code for this in the near future.

[hdgarrood]

Didn't realise I'd end up creating another issue when opening a PR, whoops. Closing this in order to discuss on #1975