Improved plugins Makefile

- It is now possible to build a plugin without adding it to the
  Makefile, by using 'make plugin.my_plugin_name'.
- To have it built through 'make all' or 'make plugins', it is still
  necessary to add it to the Makefile, as before.

Signed-off-by: Haru <haru@dotalux.com>
Improved script builtin function parser

- Removed duplicate code shared between script_hp_add and
  script_parse_builtin.
- Improved error handling in script_hp_add to be on par with the
  script_parse_builtin version.
- Added script->add_builtin function, providing a hooking point for
  plugins (i.e. text editor syntax definition generators) to obtain the
  builtin commands list.
- Special thanks to Ind.

Signed-off-by: Haru <haru@dotalux.com>