Add support for tag extension fields

[hansonw]

The ctags format supports extension fields (see http://ctags.sourceforge.net/ctags.html). These are often used to store the class names of methods, access modifiers for members, etc.

The readtags library already parses these (see tagExtensionField), so we just need to convert these to JS objects. It looks like the Tag -> v8::Object code was duplicated, so I moved the code into Tag.

Finally, I updated the README with a more complete description of the returned tag objects.

[jacwah]

Hi maintainers.

I know you are all very busy with important things. Still, I would like to make a friendly reminder about this PR and argue why it's important too (at least to me).

I want to switch to Atom in my work. But some features that are very useful in my current editor don't exist in Atom yet. One such feature is the ability to auto complete function parameters. I'm looking for a way to add this feature to autocomplete-ctags, however it requires the signature tag field, which is currently not available in the node-ctags API.

If there is any way I can help get this PR merged, please let me know!