Add support for #base and #include directives

[kerristrasz]

The #base and #include directives allow for keys to be included from other files on the filesystem, like so:

#base "another/file.vdf"

"Options"
{
    // ...
}

In the grammar, there is nothing special about #base and #include. They are normal keys. However, when the file is parsed by Valve's implementations, the included files will be merged with the source file (the exact strategy depends on which is used).

It would be nice to provide actual support for directives beyond simply permitting it in the grammar. However, it's a bit difficult to know how this would work. The parser would need to access the file system during parsing, which not every user may want. This should probably be customizable.