Groovy-6942 command args

minor cleanups and GROOVY-6942: Command args should accept arguments wrapped in hyphens.
Before:
```
groovy:000> :load 'foo 2.groovy' "bar 2.groovy"
File not found: "'foo"
```

After:
```
groovy:000> :load 'foo 2.groovy' "bar 2.groovy"
foo 2.groovy
bar 2.groovy
foo
===> null
bar
===> null
```

Also extended filename completion as a bonus. No unit tests for that one, as none existed, and they are a pain to set up requiring several files :-(