[Idea] Use CMake for building

<sup>This is just a proof-of-concept and should not be merged.</sup>

CMake-based setup is really simple (only one file vs. our current three files).
`CMakeLists.txt` is [all that is needed](https://github.com/jmendeth/hoedown/blob/cmake-setup/CMakeLists.txt), and it's *shorter* than our current Makefile.
In return, we get:

 - Proper `make install` rules.
 - Better compatibility.
 - Flexibility. A shared library is built by default, but the user can choose to produce a static archive instead.
 - Flexibility. Easily switch between *debug* (`-g -O0`) and *release* (`-O3`). At last!
 - Flexibility. The user can omit building the executable if he wants.
 - `DESTDIR` support for easy packaging (same as with Autotools).
 - Hoedown is finally able to build out of source!
 - Headers are copied to their own folder, i.e. `/usr/include/hoedown`.

Things that are still missing:

 - soversion isn't extracted from `version.h` yet, it's manually defined.
 - pkgconf package still not generated. (is it really necessary? we don't have dependencies.)

For people that want to use Hoedown but don't want to have the CMake
dependency, dropping the source files will work equally well.