worf.el: swap "a"/"h".

Now "hjkl" are like in vi: left-down-up-right.
worf.el (worf-right): improve

* worf.el (worf-regex): New variable.
worf.el: add change-heading and change-tree mode

There are now 3 (exclusive) modes that influence the arrow keys:

- keyword: move by keywords
- change-heading: equivalent to org-metaright/metaleft/metaup/metadown.
- change-tree: equivalent to org-shiftmetaright/...

Use "c" to get in to change-heading mode.
Use "t" to get from change-heading mode to change-tree mode.
Use "c" to get from change-tree mode to change-heading mode.
Use "q" to quit all modes without doing anything else (by default,
any command other than these and the arrows will quit the modes).