Add ,ld command to make a let(:double)

Want to easily make a named double for your rspec tests?
Just write your desired variable on a line, then with the
cursor over the variable, press `,ld`.

You can easily convert:

    local_vari*able_foo

to:

    let(:local_variable_foo) { double(:local_variable_foo*) }

where `*` represents your cursor.

Notice that you end up in a great position to add
key/value pairs!
Merge pull request #36 from mkenyon/add-let-double

Add ,ld command to make a let(:double)