Python API support

[KiwifruitDev]

Is your feature request related to a problem? Please describe.
When developing StageHammer, I could not find any documentation related to using this add-on from another script.

Is your feature request related to a specific game? Please specify.
StageHammer depends on Source SDK 2013 Multiplayer.

Describe the solution you'd like
Having the ability to import a VMF directly from Python without implementing a workaround would help solve this issue.

I believe that a simplified method to select game directories and VPKs outside of vmfpy would also help, as otherwise a script would have to manually create a VMFFileSystem instance.

Additional context
My use case for a Python API is to use Blender and io_import_vmf as a means to convert the map's geometry to Collada DAE format easily.