Add SelectNone action

which clears current selection. Also make C-g the default key binding
for this.
Add SelectVisible action

which selects every match that is currently displayed on screen. To
determine which matches are currently visible I chose to expose
currentPage (index, offset, and page size) through the Context. This is
updated every time the screen is redrawn.
Add SelectAll action

which selects every match, visible or not.
Merge branch 'select-none-and-all' of github.com:jowl/peco into select-none-and-all

Conflicts:
	ctx.go
s/register/Register/g
Merge pull request #128 from peco/select-none-and-all

Select none and all