Force file overwrite

If people use the `NO_CLOBBER` option, the config file will not be overwritten, unless you use `>!` or `>|`. (I use `NO_CLOBBER`)
Fix whitespace issues

Some whitespace were tabs, some were spaces
Add checks: if config file exists and is writeable
Fix `wd add`: bug when no arg is given

Running `wd add` (with no argument) would add an entry called "false"
pointing to the current directory.

This was because `$2` was empty and thus "false" becomes the name of the
warp point, instead of being the "forced overwrite" argument.
Added 'show' to help
Added completion feature
Add completion
Merge branch 'malthe-completion'

Conflicts:
	_wd.sh
	wd.sh