Added options for METABUILD and PKGBUILD relocation

The script has been added two options:

+ `-m`, `--metabuild`: Allows the `METABUILD` file location to be specified, defaulting to the old value.
+ `-p`, `--pkgbuild`: Allows the `PKGBUILD` file location to be specified, defaulting to the old value.

This will allow `metapkg` to be used from scripts without having to use lots of temporary directories or heavy renaming.