add yas-oneshot for quick snippet creation

[lewang]

I had this laying around in my personal config for a long time, thought others might find it useful.

Documentation included. This does something similar to #309, but allows for the possibility of adding placeholders.

[joaotavora]

This is very interesting after playing around with it. I would like to see this integrated in yasnippet.el.

• I like a lot that yas-oneshot varies behaviour depending on whether there is a region selected or not. Also like the name 👍
• it is missing more documentation. It should pass the M-x checkdoc command.
• Need some unit tests for the basic cases. See yasnippet-tests.el
• invoking yas-oneshot should highlight the symbols selected for placeholders in the active region. If this is difficult to implement, don't worry a lot about it.
• There should be a yas-offer-to-save-one-shots customization variable (and this would mean integration with New snippet with selected text #309). When set to t and when the user is done, it should pop to a new-snippet buffer, with the snippet neatly setup and ready to press C-c C-c for loading the snippet into whichever mode the user chooses.
• Also the messages presented to the user should be a little clearer and be preceded by "[yas]". It should be clearer that inputting nothing means "I'm done"
• Discovered that there was a serious bug in yas-reload-all, where it reset yas-minor-mode-map.I fixed this is 1ef0091, so now you can use eval-after-load. However, if this is to be integrated in yasnippet.el it is not needed.

[lewang]

Thanks for checking it out. Yous points all seem valid, I'll make the changes soon.