* doc/snippet-development.org: New "Indentation markers" section.
* yasnippet-tests.el (indentation-markers): New test.
* yasnippet.el (yas--indent-markers): New variable.
(yas--indent): Use it instead of searching for $> directly.
(yas--indent-parse-create): New function, records occurences of $> into
`yas--indent-markers'.
(yas--snippet-parse-create): Call it.