Parsing and handling priority

[hikerpig]

See UltiSnips's doc

A line beginning with the keyword 'priority' sets the priority for all
snippets defined in the current file after this line. The default priority for
a file is always 0. When a snippet should be expanded, UltiSnips will collect
all snippet definitions from all sources that match the trigger and keep only
the ones with the highest priority. For example, all shipped snippets have a
priority < 0, so that user defined snippets always overwrite shipped snippets.