Allow lilac to be installed as a system package

[yan12125]

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 (write a setup.py #35 (comment))
• ~/.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:

• ./lilac lilac-git from a git clone
• lilac lilac-git with the package created by PKGBUILD above

/cc @Xuanwo @oldherl

[lilydjwg]

Some of these are pointless. Please don't add entries defensively. Changes are expected to be reviewed before committing so any unintential additions should be noticed.

[yan12125]

Maybe just remove those entries? Some of those entries are for generated file from python setup.py install, python setup.py sdist or python setup.py build. But I guess those setup.py commands will not appear during development of lilac?

[lilydjwg]

setup.py generates only dist, sdist and build. I don't know where others come from.

[Xuanwo]

They are generated by IDEA and cloud be deleted if not used.

[yan12125]

*.egg-info is generated from python setup.py egg_info (a command used by python setup.py build or python setup.py install).
.eggs is used to store packages specified in setup_requires if they are not installed beforehand.

In the updated commit (6f2479f), I keep entries that might be generated by setuptools and remove others.