Add module directory to include path

[lrascao]

Several projects use an include path relative
to the project's root.
file:compile will look in three places for the include
files:
The current working directory
The directory where the module is being compiled
The directories given by the include option

[ferd]

We're gonna need a test with that to avoid accidental regressions. There's a basic explanation on how to write them in https://github.com/rebar/rebar3/blob/master/CONTRIBUTING.md#tests, but if you want some help, I can try to write something up at some point this week.

Otherwise the changes look good.

[lrascao]

Sure, i'll try and get something done this week

[lrascao]

2 new tests added

[ferd]

Sounds good, +1.