Add support for global opts file - fixes #768

FileSystem.GlobalOptsFile will return the path for a scriptcs.opts file
in the ModulesFolder

ArgumentHandler looks for non-default option values first from the
command line, then from a local opts file if it exists, and finally from
a global opts file if it exists.

Rename ModulesFolder to GlobalFolder per discussion on #768

Update ShouldUseScriptOptsIfParsingFailed: parser mock will create a
ScriptCsArgs instead of returning null to avoid null reference exception
Added constant for config filename
Add fix to pull out the inner exception when an AggregateException is returned from Roslny
Merge pull request #773 from ryanrousseau/opts2

#768 Add support for global scriptcs.opts file
Adding same fix for mono
Merge pull request #834 from glennblock/dev

Add fix to pull out the inner exception when an AggregateException is returned from Roslyn
Added a code segementer (lexer, parser) for the mono engine