Add howdoi
Add howdoi

Motivating example for merge approval:
```
$ howdoi git revert file to master
git checkout filename

$ howdoi bash check for empty file
#!/bin/bash -e

if [ -s diff.txt ]
then
        rm -f empty.txt
        touch full.txt
else
        rm -f full.txt
        touch empty.txt
fi
```
unixorn/terminals-are-sexy,k4m4/terminals-are-sexy