Introduce a new non-bundling compilation mode

[Renegatto]

Currently purescript-lua only support bundling, I.e. merging all the modules into the one lua file.
However it is often needed to keep the initial directory structure and keep lua modules small.

So the proposal is to add a new compilation mode (--module?) that would preserve source tree structure in a filesystem level.

This compilation would look like this:

foo/Foo.purs
foo/Bar/Baz.purs

->

foo/Foo.lua
foo/Bar/Baz.lua

Or something like.

Note, that there are may be some pitfalls to consider. See:
TypeScriptToLua/TypeScriptToLua#1445