Add CMake install rules

When using some generators (like NMake or Ninja) that don't
use the MSVC IDE, the files aren't installed anywhere, as
the step copying these to the Output/ directory is only true
with MSVC_IDE (including MSBuild from the CLI).

Even in the event that copying to Output/ was made applicable
to non-IDE builds, normative install rules are also going to
be necessary for any future cross-platform concerns. Best to
get it done now.

Include files go in ${CMAKE_INSTALL_PREFIX}/include/avisynth,
both to keep /include cleaner, and because that's already the
target path for avisynth_c.h in Libav.

Plugins get installed to ${CMAKE_INSTALL_PREFIX}/lib/avisynth,
to match both AvxSynth's and VapourSynth's plugin directory
path conventions, even though this means nothing on Windows,
and given AviSynth+'s support for multiple plugin dirs, this
would merely be the suggested place for system plugins.
Add CMake uninstall target

Copy-pasted from CMake's FAQ:
http://www.cmake.org/Wiki/CMake_FAQ#Can_I_do_.22make_uninstall.22_with_CMake.3F