Allow lilac to be installed as a system package

Based on #78. Fixes #35. For archlinuxcn/repo#1651.

Some implementation notes

I use setuptools-scm to generate the Python package version
The Python package name is archlinuxcn-lilac (https://github.com/archlinuxcn/lilac/issues/35#issuecomment-662251124)
~/.lilac/config.ini is another path for the configuration file
A proof-of-concept PKGBUILD can be found at https://github.com/yan12125/repo/tree/3d74b0b64d5c9aaa20f89aa1bd38e86969a59fae/archlinuxcn/lilac-git
config.ini.sample is not handled by setup.py. I believe it should be installed in /usr/share/doc. However, someone may want to install this package with python setup.py install --user, so I leave it to downstream packagers. See PKGBUILD above as an example.

Tested scenarios:

/cc @Xuanwo @oldherl

archlinuxcn / lilac