Allow `sh` as an alias to `shell` or `bash`

GitHub Flavored Markdown allows the "sh" language designator when using
fenced code blocks, and this essentially maps to "shell" or "bash". e.g:

```sh
export EDITOR=vim
command git commit -asm "Initial commit."
```

Signed-off-by: David Celis <me@davidcel.is>
Merge pull request #51 from davidcelis/master

Allow `sh` as an alias to `shell` or `bash`