Vim support for Pyxl

This includes filetype detection, syntax hilighting, and indentation.
The current code has not been thoroughly tested, but auto-indenting a
few real-world Pyxl files led to basically reasonable results.
Changes to vim syntax file

Fixes issue where <= is treated as the start of a pyxl object.
Allows _ in tag names
Adds frag and if to the list of recognized tags
Merge pull request #1 from PollRobots/master

Changes to vim syntax file
Merge pull request #2 from jboning/vim-syntax

Vim syntax