Add TeX plugin

[TimothyGu]

Matches environment \begin/\end.

[joaoseckler]

Nice patch. This should really include \( and \[, though:

 let b:pear_tree_pairs = extend(deepcopy(g:pear_tree_pairs), {
-            \ '\\begin{*}': {'closer': '\\end{*}', 'until': '[{}[:space:]]'},
-            \ '$$': {'closer': '$$'},
+            \ '\\(': {'closer': '\\)'},
+            \ '\\[': {'closer': '\\]'},
             \ '$': {'closer': '$'}
             \ }, 'keep')