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.