Bookmark formulae added

This is a very handy functionality that lets us bookmark folders, and easily navigate to them in the future, rather than manually creating aliases for navigating into each folder.

### Sample Use Case

```bash
cd /webapps/app1
bookmark firstapp
```

then next time  you could directly do
```bash
go app1
```

rather than having to cd into the folder manually. 

So basically list of commands are -: 
a) bookmark [name] - By default, it takes the folder root
b) unbookmark [name] - Same principle
c) go req1

A few people suggested that it would be great to add this to brew -: https://github.com/mathiasbynens/dotfiles/pull/405, so I gave it a shot. What do you guys think?