added AliasCommand for REPL command aliasing
added REPL command test file and changed the initialization in REPL
added behavior test
addressed code review feedback
Merge pull request #769 from filipw/feature/alias-repl

Allow REPL command aliasing
prefixed FileSystem locations with scriptcs_
changed command line to assume REPL when no command or script name is passed
- this is to allow running modules without having to specify -repl, e.g. scriptcs -m "migrate"
added migrate module