Support for Git Sources and Build Commands

[odysseywestra]

Styrene should have support for git sources and build commands. Having this would allow a packager or developer to be able directly test their code without having to use an external tool like makepkg-mingw and a PKGBUILD file to make the package. Plus it keeps the code duplication down as well. Here is an example for MyPaint:

[mypaint-git]
url = https://github.com/mypaint/mypaint
path = ../  # <-- For example if the cfg file was stored in /styrene.
build-packages =
    {pkg-prefix}setuptools
    {pkg-prefix}build-package-two
    {pkg-prefix}build-package-two
command =
    python setup.py build
    python setup.py install --root="{install-root}" --prefix="{install-prefix}"

[libmypaint-git]
url = ...
path = ...

This would also mean we would need to add new substitutions like install-root and install-prefix so styrene would know where to install the files.

In the bundle field, we should create a dynamic substitution that reflect the source name. That way styrene, can also glean the default metadata off the first like you can do with standard packages.

[bundle]
packages = 
    {mypaint-git}
    {libmypaint-git}
    {pkg-prefix}gtk3