Make `outshine-self-insert-command` compatible with `delete-selection-mode`

Fixes #25
Merge pull request #26 from ffevotte/delete-selection

Make `outshine-self-insert-command` compatible with `delete-selection-mo...
First version of `outshine-use-outorg' added.

Since Org-mode and Outshine are just two views on the same file, and
Outorg allows to switch quickly between the two views, this function is
a first attempt to make the full Org-mode functionality available in
Outshine source-code buffers without reimplementing Org-mode.

See the Outshine speed commands for usage examples.
Use outorg functions slightly refactored.
outshine--set-outorg-last-headline-marker function improved.
Added new outshine-speed-commands with 'outshine-use-outorg'.