Enhanced Tweelexer and external passages.
* Populate externalPassages set using existing buildIncludes method.
* Lexer highlights link to found external passage as good.
* Remove found external passages from broken links list.
* Dont show broken link emblem if all internal and external passages
found.
Merge pull request #128 from greyelf/i014_tweelexer_ext_passages

Tweelexer and external passages (initial effort)