Add support for #include, #base declarations

[StefanH-AT]

https://developer.valvesoftware.com/wiki/KeyValues

basically:
#include appends another file to the current one
#base merges another file into the current one (replacing existing key/value's)

I haven't seen them in use yet either, but it's a core feature of valve's keyvalue files (unless they removed it later on)

https://github.com/ValveSoftware/source-sdk-2013/blob/master/mp/src/tier1/KeyValues.cpp#L2238
https://github.com/ValveSoftware/source-sdk-2013/blob/master/mp/src/tier1/KeyValues.cpp#L2325