Fix the docstring for ShellCodeToken

It's not vim-syntax-related, but it was bugging me.
fixup! Clean up miscellaneous syntax items
Rewrite parsing for tab stops
Tweak snippet/global highlighting to always highlight the end

The snippet/global regions were always ending correctly, but an unclosed
contained region could cause the wrong highlighting to apply to the
endsnippet/endglobal. Tweak the definitions such that the snippet/global
regions actually end at the start of the line and the footer occurs
outside the region.
Remove the coloring for python commands

Python commands tend to be longer than normal commands, and they already
include embedded python syntax coloring, so coloring them red like shell
commands is a bit overwhelming.

Preferably there would be some subtle background color we could apply,
but I don't see any pre-existing group name for that, and defining our
own would not play well with colorschemes.
Stop embedding python/viml/sh if the `` includes {

In order to handle brace balancing properly, we need our own region to
match the {/} pairs, which unfortunately means that if a { occurs in a
command substitution, we have to disable the embedded language
highlighting.