Add tab completion for paths in the REPL

[jvns]

In the "nice-to-have" category -- tab completion for paths would be useful. Maybe when the new REPL is written in 0.3.

IPython does this inside strings, so it tab completes when you start with "./, for example.

(see also: #4477)

[simonster]

Related to #3392. (I disabled the confusing and mostly non-functional path completion for any unrecognized token in #4361.)

[ivarne]

I don't want to discourage this effort, but it as this feature require a lot of testeing to work on different platforms, it might be useful to track this issue in https://github.com/loladiro/REPL.jl and https://github.com/loladiro/REPLCompletions.jl. That way the switch will give more features, in a form that is easier to utilize in iJulia and other coding platforms.

[jvns]

That makes sense! Thanks @ivarne. I guess it would make more sense to make an issue in REPLCompletions?

[JeffBezanson]

dup of #3392.