RFC: include and exclude files by shebang contents and other criteria

[tomeon]

Intended for cases like scripts, which may lack extensions indicating their file/language type.

An incidental change is the introduction of the format.Matcher interface and various types implementing this interface. The idea is to encapsulate various bits of logic:

1. Does this matcher match the supplied path (for the matcher-specific sense of "match")?
2. Should this matcher be ignored (e.g. because it has no configured glob patterns)?
3. Is this matcher for including or excluding files?

I do not have very much experience in Go, so I'm not certain whether this approach is a good idea or, if so, whether it is implemented sensibly. The intentions are:

1. To simplify the use of matchers (assembling a CompositeMatcher is a bit verbose/unwieldy, but using it is IMO straightforward -- one call to Wants(path) for each path under consideration), and
2. To provide a basis for implementing additional inclusion/exclusion logic.

Thanks in advance for your consideration!