Adding workaround to TabExpansion2 for completion of native options

For some reason, PowerShell does not call the registered completer for
native commands if the word to complete is - or --.
Reworking completer to use the ast extend to find the right thing to complete
TryNativeCommandOptionCompletion should only look for StringConstant options
Using only the filename without extension for native command lookup
Cleaner loging in ast find predicate in TryNativeCommandOptionCompletion
Merge pull request #25 from powercode/NatOptBugFix

Adding workaround to TabExpansion2 for completion of native options