Implemented the fileParser and added DynamicExamples.

Main additions:

- Implemented `Parsers.fileParser` (removes a TODO)
- Added `Parser.DynamicExamples` (lazily generates/regenerates completions; especially useful for file completions if there are a lot of files in subdirectories)
- Added `Parser.SourceOfExamples` (an abstraction of the set of examples; allows lazy listing of examples; the only alternative before this was `Set[String]`, which cannot be lazy; `SourceOfExamples` is used by `DynamicExamples`)
- Added `Parsers.FileExamples` (searches for files/directories recursively; it should be fairly efficient, as it lazily traverses only the path with the user's prefix; if there's no prefix, then only a limited number of files are displayed&mdash;this avoids long recursive traversals)