Add exclude-private option

[ascarter]

Added flag -exclude-private to chose to filter any symbols that are not public when building the tag file. This is very useful for building a tag file for the Go standard library or a third party API.

[jstemmer]

Instead of checking p.excludeTag everywhere, you could replace all the p.tags = append(p.tags, tag) instances with a function p.add(tag) that adds a tag to the tags slice when it shouldn't be excluded.

[jstemmer]

You could also just add an extra file in tests/, add a excludePrivate as a field in the testCases struct for this specific option and then test if the correct (exported only) tags are returned.

I understand why you'd want to do it this way, as this will check all the different types of tags in all the testfiles. However, I think I'd prefer not to treat this option as a special case compared to the other tests. What do you think?

[ascarter]

Good point - was attempting to avoid redundancy but you are right - that it isn't a special case. Will change the tests.

[ascarter]

Made requested changes and squashed commits

[ascarter]

@jstemmer Just wanted to remind you that this is ready to go.

[jstemmer]

@ascarter a quick update: I'm in the middle of changing jobs (and moving to another country), so I haven't had time to check on my github projects. I should have a laptop and some time soon. I appreciate the time and effort you put into these PR's, sorry to keep you waiting.

[ascarter]

@jstemmer - any chance you might be able to merge the two pending PR's I submitted? Would love to have this in there instead of managing my own fork.

Thanks!