README.md: Fixed brew install instructions
Anchor gitignore paths

This makes the intention more explicit and avoid any accidental
matching elsewhere in the tree.

Conflicts:
	.gitignore
Rename and refactor mch_FullName.

* Rename mch_FullName to mch_full_name to match the style guide.
* Add mch_full_dir_name, which saves the absolute path of a given
directory relative to cwd into a given buffer.
* Add function append_path, which glues to given paths with a slash.
* Adapt moonscript coding style to the tests.