Fix highlighting of Python/VimL commands

Also mark all the highlighting as default, so it can be overridden more
easily by the user.
Fix embedded VimL highlighting

The b:current_syntax variable is set by syntax-include, so it needs to
be unset again or the second syntax-include does nothing.
Fix syntax name

b:current_syntax should be set to "snippets", as that's the filetype,
not "snippet".
Properly handle \` escapes in commands
Fix coloring of shell/VimL/Python command substitutions

Include the sh syntax for shell commands.
Make the matching on !v/!p work more accurately.
Highlight the !v/!p separately from the embedded Python/VimL.

AFAIK the only issue now is that "\`" escapes inside shell/VimL/Python
are not treated as bare "`" characters by the embedded syntax, but I
don't believe there's any way to fix that.
Add hierarchy to syntax file

Redo comments with fold markers to provide an appropriate hierarchy to
the syntax file itself.
Rewrite extends/priority/clearsnippets
Mark up syntax items with 'display' as appropriate
Rewrite syntax matching for `snippet`
Rewrite syntax matching for `global`

The body of a global now only has Python syntax coloring if the trigger
is !p. This matches the actual behavior of UltiSnips. AFAIK right now
globals with other triggers are useless, but this opens the door to
proper syntax coloring for those if they ever gain a use.
Fix empty snippets/globals
Syntax highlight the snippet trigger properly

This supports multiword triggers, docstrings, options, and even
highlights invalid multiword triggers as errors.
Clean up miscellaneous syntax items

Remove a few obsolete items.

Remove the snipTabsOnly item. A tab-only line is not necessarily an
error.

Color leading spaces with a red background, because the default Error
coloring doesn't affect the display of spaces.
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.
Set up basic highlighting for snipmate files

Try to detect a snipmate-formatted file by looking at the directory name
and searching for endsnippet. If we think it's a snipmate file, switch
over to an alternative syntax highlighting variant.