Add initial scripting support

Clean up crossbuild scripts a little

Console scrollback now uses a GString instead of a buf_t; probably gonna
slowly move away from the custom data structures and use GLib's

Refactor the console's input and scrollback a little; move more members
into input/scrollback

Add C_Echo and C_MEcho commands, for when you don't want any formatting

Fix some console layout issues

Use 'fmt' instead of 's' in doom_pprintf

Fix doom_printf's prototype in g_game.h to use named parameters
Add shorthand console command functionality