Adding functionality to bookmark folders

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 /Applications/mywebapps/app1
bookmark app1
```

then next time  you could directly do
```bash
go app1
```

rather than having to cd into the folder manually. 

P.S. - Been using this functionality for a long time, can't remember its original author. Code's been heavily modified by me now. :-)