Table tags no longer self-closing. Rows and cells now contain their
contents. Also refactored out an `emit_table_tag` method.
Note: this will require changes to the Tag node and possibly the builder,
those changes will be in the next commit.

For C compatability, switch table cell end to return the stack.
Now context is kept by using `keep_context` when calling `self._pop()`.

Various changes to avoid returning tuples - working on the C tokenizer
made me realize this was a bad idea for compatability/similarity between
the two.

Added comments, tried to keep to 80 character lines.

Started parsing table support and added the start of table support.
This is a big commit (ugh) and it should probably be split up into
multiple smaller ones if possible, but that seems unworkable as of
right now because of all the dependencies. Also breaks tests of
CTokenizer (double ugh) because I haven't started table support there.

May want to pick line by line on this commit later but I need to save
my work for now.

All tag tests passing. Still need to finish backslash support and
support for templates and tags within <open> tags.

- Translation dict, contexts, parse_* and handle_* hooks in tokenizer.

* Some other fixes, additions.