Add simple options for beginning and end of onion url

Adds feature requested in #3 to make it easier for someone to make a .onion URL without regular expressions.

Use -b to put at the beginning (```./shallot -b name```same as ```./shallot ^name```)
Use -d to put at the end (-e is already used) (```./shallot -d name```same as ```./shallot name$```)
Use -a to put anywhere (```./shallot -a name```same as ```./shallot name```)