Added extra options to exclude symbols, and local auto-generated cross-references

Hi,

I want to use appledoc to generate documentation for a large project, and it works very well (thanks for writing it!).

I had two issues, which this pull request addresses.

1. The project has some categories with the name 'Deprecated', which should not appear in the final documentation. Doxygen has an option to ignore symbols (as well as files). This change adds an argument '--ignore-symbol', which takes a 'glob' argument (e.g. '*Deprecated*'). If that glob matches any class, protocol, extension or category, then that symbol will not appear in the final output.

2. As other people have found (reading your past issues), sometimes appledoc can create false links to local symbols if those local symbols are common words such as 'and'. However, auto-linking of cross-references is very useful. I tried the explicit-crossrefs argument, but that had several problems. Instead, This change adds an argument '--require-leader-for-local-crossrefs', which works well for my case, and I think it is useful in general. When this is supplied as an argument, auto-linking of local cross-references only happens when the symbol name appears with a leading '-' or '+'

I have also added two test cases for the new arguments.

Best Regards
--Anthony