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

Add 'aw', 'a"', 'a\'', 'a(', 'a{', 'a>' keymaps.
Add 'select-a-word', 'select-a-double-quotes', 'select-a-single-quotes',
'select-a-parentheses', 'select-a-curly-brackets', 'select-a-angle-brackets'
commands.
Add specs for all keymaps.
Changed 'select-a-*' to 'select-around-*' for appropriate comands.
Fix 'select-a-word' command not behaving correctly when cursor is on the first character of a word.
Add square bracket and back tick text objects.

Add 'select-inside-square-brackets', 'select-inside-back-ticks',
'select-around-square-brackets' and 'select-around-back-ticks' commands.
Add 'i[', 'i`', 'a[', 'a`' keymaps.
Add specs for all commands.
Add aliases for text object commands.

Add 'i }', 'i B', 'a }', 'a B', 'i >', 'a >', 'i ]', 'a ]', 'i )', 'i b',
'a )', and 'a b' keymaps.
Merge pull request #373 from anthonysandrin/master

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