Improve discoverability of Sublime's functionality

Sublime Text (both 1 and 2) are packed with great functionality, and packages add much more. However, it often takes a web search to find out about some of Sublime's more obscure features.

Improve the discoverability of Sublime's manifold functionality. Some ideas:

* Optional tips-on-startup dialog (with "don't show next time" checkbox); especially useful for new users of the demo version

* Add a mechanism like `apropos` to search for commands/keybinds that may do what you want. Having a short plain-English description with each command would improve search discoverability. Descriptions would need to be associateable with both commands and specific keybinds. Add a help menu item labeled "I want to..." to open the apropos prompt.

* Add a mechanism for the user to get context-sensitive help for any keyboard command or menu item. Wire it up to a wiki to let the community help build up these docs.

* Add a help menu item "Keyboard shortcuts" which generates a human readable list of the current keybindings in a new buffer. This list could sort the binds by keystroke and include the short `apropos` description for each keybind.

* Require installed packages to provide apropos-style descriptions of all their commands and key bindings.

* Allow package creators to include a help file with their package. Make these help files accessible through a new help menu item "Package Help >".