Issue #414 - snap package compatibility

PR #391 changed the locaton into which system-wide cheatsheets are
installed to `/usr/share/cheat`, in order to comply with FHS. However,
this is causing conflicts with the `snap` packaging process.

This commit removes hard-coded references to `/usr/share/cheat` (outside
of `config/cheat`), and instead reads the cheat path via the
`CHEAT_PATH` config value (which may be set either in `cheat/config`, or
exported as an environment variable).

Lastly, this commit makes `setup.py` "aware of" the `CHEAT_PATH` env
var, allowing us to specify to where sytem-wide cheatsheets should be
installed during the `snap` build.

cheat/sheets.py

@@ -63,7 +63,6 @@ class Sheets:
         """ Assembles a list of directories containing cheatsheets """
         sheet_paths = [
             self.default_path(),
-            '/usr/share/cheat',
         ]
 
         # merge the CHEATPATH paths into the sheet_paths