Implement command line option to set file extension for compiled files

[thekid]

Default:

$ xp compile HelloWorld.php dist/
# ...

$ ls -al dist/HelloWorld*
-rwxr-xr-x 1 timmf timmf 519 Feb 19 13:12 dist/HelloWorld.class.php

Using .php file extension:

$ xp compile -e .php HelloWorld.php dist/
# ...

$ ls -al dist/HelloWorld*
-rwxr-xr-x 1 timmf timmf 519 Feb 19 13:13 dist/HelloWorld.php

See #158 (comment)