Add 'a' operator-pending commands. Fix for issue #369.

Add 'select-a-word', 'select-a-double-quotes', 'select-a-single-quotes', 'select-a-parentheses', 'select-a-curly-brackets', and 'select-a-angle-brackets' commands.
Add 'aw', 'a"', 'a\'', 'a(', 'a{', and 'a>' keymaps.
Add specs for all keymaps.