issue 33.
* Added functions `yas/field-undo-before-hook',
`yas/field-undo-after-hook', `yas/field-undo-group-text-change' and
variables `yas/field-undo-history',
`yas/field-undo-forbidden-commands' to implement field-level undo.
* Modified `yas/cleanup-snippet' to record the actual snippet end
in the snippet before cleaning up the main overlay.
* New hooks `yas/field-undo-before-hook' and
`yas/field-undo-after-hook' added to `pre-command-hook' and
`post-command-hook', respectively.
* Slightly changed the commentary of the "Snippet register and
unregister routines section"
* Modified step 12 to call `yas/undo-expand-snippet' without the
`(point-min)' argument, which led to incorrect undo behaviour.
* Added a new step in `yas/expand-snippet' describing the local
restore and save of undo infomation in a new slot of the
`yas/snippet' structure. This is step 14, previous 14 was moved to 15.
* Modified `yas/undo-expand-snippet' to kill text up to the the
correct snippet end, which was not being calculated correctly. It is
now a slot of the snippet itself and no longer a parameter.
* Added functions `yas/current-field-text',
`yas/replace-fields-with-value',
`yas/remove-recent-undo-from-history' to refactor some code used
twice or more.
* Modified `yas/check-cleanup-snippet' to detect trigger cleanup
if point exits any primary snippet field, of any
group. Previously cleanup would only be triggered on exiting
all groups. This is a follow-up to issue 28.
* Added two slot to the `yas/snippet' structure to allow for
field-level undo and correct small bug in the previous fix of issue
28.
* Modified `yas/cleanup-snippet' to call
`yas/after-exit-snippet-hook' as a follow-up to feedback on
issue 28.
* (Accidentaly :) changed (but corrected!) indentation of most
existing functions. This was done by a buffer global indent-region.
* (Accidentaly :) ran `checkdoc' but then gave up, it's
uncessary. Didn't change much stuff though.
