New option for brew install: --only-dependencies

The traditional approach to installing the dependencies of a formula
is the following:

~~~
brew install `brew deps formula`
~~~

This approach ignores any options that are specified in the parent
formula. This pull request adds an `--only-dependencies` option to
brew install that installs the dependencies of a formula with
optional flags, but returns before installing the parent formula.

EDIT: changed from `--dependencies-only` to `--only-dependencies`.