added error on let statement not directly within block

Signed-off-by: Bernard `Guyzmo` Pratz <guyzmo+github@m0g.net>
fixed style and verb
updated no let within a block stmt so it does not put the variables in environment ; added more tests
added A LOT of tests
renamed isMozOrESNext to isAfterESNext;
added isAfterES5;
refactored ESnext/Moz support to raise errors on future features;
fixed several parsing issues

Signed-off-by: Bernard `Guyzmo` Pratz <guyzmo+github@m0g.net>