[inaka/elvis#96] Elvis plugin.

Running `make elvis-rock` does the following:
- If there's no existing `elvis.config` file, the default one is downloaded.
- If the `elvis` executable script doesn't exist, it is downloaded in the current directory.
- The `elvis rock` command is executed.

This PR closes this issue in inaka/elvis#96.
Any reason for the 2 functions since you only call them once? Doesn't seem very useful on first look.