Add support for the BEX extended string syntax.

Boom added an alternate method for overriding Dehacked strings, using
the [STRINGS] section with special symbolic names for each of the
strings that can possibly be replaced.

This format is used in the Freedoom DEHACKED lump (deliberately, so
that the Freedoom WAD does not include the original text to be
replaced). As we want to have support for the Freedoom IWADs with
Chocolate Doom, add support for the BEX [STRINGS] section, but
protected by a magic comment that must be included in Dehacked/BEX
files that use it.

Thanks to Fabian Greffrath for implementing this.