Initializing "file" local var so as to avoid explosions

that are probably confusing and scary for folks unfamiliar with Ruby
backtraces.

Steps to reproduce:
````` bash
# in the haste-client working copy
ruby -I.:./lib bin/haste foo
/Users/d.buch/workspace/haste-client/lib/haste.rb:15:in `initialize': undefined local variable or method `file' for #<Haste::CLI:0x007ff25b96eab0> (NameError)
        from bin/haste:7:in `new'
        from bin/haste:7:in `<main>'
`````