This commit adds support for the frege intellij plugin by

 - exposing the frege compiler jar as a dependency, so the plugin can
   recognize the standard library.
 - adding the name 'fregeRun' as that is used by the plugin to run files
 - add support for overriding the main module executed by runFrege so
   that arbitary files can be run by the plugin

Frege Compiler Make Mode Test

The following applies if the frege compiler is run with the `-make`
[flag](https://github.com/Frege/frege/wiki/Compiler-Manpage#make-mode):
Given two dependent frege files. If the `mainSourceDir` property is not
correctly configured, then the frege compiler cannot find the dependent
frege file and will therefore fail the compilation.

The frege compiler starts at the `mainSourceDir` and searches
recursively for dependent modules. E.g. If `mod.Main.fr` imports `other.Dep.fr`
then the frege compilers searches for `mainSourceDir/other/Dep.fr`.

Renames Configuration: implementation -> frege

The name implementation is already used by the java plugin and it
leads to conflicts when combining both plugins.

Adds testFrege Task

Adds initFrege Task