Magic configuration for bash completion

Some bash magic to lazy load and configure bash_completion for nit.

Steps for a functional installation:

0. [if you have old binaries] `$ rm c_src/nitg`
1. `$ make`
2. [optional], put the `bin` directory in your PATH
3. `$ source misc/bash_completion/nit`
4. [optional] link (or copy) `misc/bash_completion/nit` in `/etc/bash_completion.d`, or source it in `~/.bash_completion`
5. profit