Expose fields for the Node object

This commit creates `tag.fields` functionality, where this returns the tag extension fields as specified by the ctags format and as available in the `fields` member of the `tagEntry` struct. This gets the supplied tag object in node to hold all information that a C `tagEntry` would.

It includes some additions to the README as well concerning the available `kind` property for the tag object, as well as some other tiny things.
Why was this change needed?