Specify extra source paths

[jsoo1]

This allows specifying extra source directories from the command line:

$ elm-analyse Main.elm other/Main.elm bin/Main.elm

This would fix the underlying problem in #214 and the direct issue in #93

It would also make elm-analyse easier to use on small projects with only a single file. It also will allow checks to be run on Main modules outside of source-directories.

Rationale for the change:

• elm.json does not allow specifying a Main module, and the Main module may not necessarily live in a source-directory. It instead might be specified specified when using elm make.
• elm-analyse does not currently do analysis on an entire project because of this limitation. The idea here is to allow for more complete analysis when there are multiple main modules or just a single module outside of a source directory.
• Using the unnamed cli parameters mirrors the way elm make is used. Compare:
  elm make:

$ elm make Main.elm

elm-analyse:

$ elm-analyse Main.elm

So by convention, elm-analyse should fit into the user's way of working with other elm tools.