New methods which operate on zenkaku chars which have western (ASCII) counterparts:

Hi, firstly GREAT gem--extremely useful.

I've added a new methods:
- `ascii_zen_to_han`
- `ascii_zenkaku?`
- `contains_ascii_zenkaku?`

These operate only on chars which have western (ascii) counterparts, specifically Ａ-Ｚ　ａ-ｚ　０-９　and symbols like ＠？！ (but **NOT** Japanese symbols like 。「」、 according to the definition in Moji). I added this because I often want to convert these guys while leaving zenkaku katakana as-is.

Readme / specs updated as well. Also added a few extra specs for existing hankaku / zenkaku methods.

I don't think there's a use case for the inverse `ascii_han_to_zen` as regular `han_to_zen` should suffice.