Allow default keymappings to be changed with simple configuration

[andrewferrier]

https://www.reddit.com/r/neovim/comments/1099agb/comment/j3x5w0w/

[andrewferrier]

Re-reviewing this I've decided to leave this as-is. There is already a a way of customizing keymappings that's well documented for advanced users, and for novice users the out-of-the-box keymappings are OK. Although I could add a mappings table as per the Reddit discussion, I've also been convinced that there are advantages in the other direction (e.g. for folks who like to centralize their keymappings).

That, combined with the 'if it ain't broke don't fix it' principle, means I'm going to not implement this unless I get specific feedback that people want it and prefer it.

[andrewferrier]

I'm reopening this for implementation; this has caused confusion twice now because of the need for <expr> mappings; see #75 and #66. Let's do this as an OOB default, maybe maintaining the older approach (deprecated) for backward-compat.