Support local installs (via `yarn add`)

[wincent]

yarn add git-cipher

Will let you run commands like this:

yarn run git-cipher init
yarn run git-cipher unlock
yarn run git-cipher add foo.txt

But clean/smudge (etc) filters won't work because git-cipher init sets them up to run:

git-cipher clean %f

ie. it assumes a global install.

So, we should support that, and teach init to instead configure things to run as:

yarn run git-cipher clean %f

(etc)

Or perhaps:

$absolute_path_to/node_modules/.bin/git-cipher clean %f

Or, teach the globally installed git-cipher to look for a local copy and delegate to that if present.