add feature: user should not change the pronunciation of an alphabet

[harryoooooooooo]

I add the feature discussing in #79.

Just check if the size of phrase and bopomofo are both 1, and also they are not the same,
then the phrase shouldn't be a bopomofo.

[coveralls]

Coverage remained the same at 94.915% when pulling bcc6b54 on harryoooooooooo:chewing-editor-issue79 into 1679266 on chewing:master.

[davy-tw]

What if I typed "ㄅㄉ" with bopomofo "ㄇ ㄋ"?
This case also try to create mistaken bopomofo of these words.

[harryoooooooooo]

I think we should not modify the pronunciation of a single alphabet.
However, a combination of alphabet maybe occur a special meaning, so that user want to make it a phrase.
we can discuss how chewing-editor should deal with it and have a consensus.

[davy-tw]

The truth is though we added a phrase with impossible bopomofo, chewing still not provides these phrase according to its built-in bopomofo mapping.

So even we added a phrase "ㄅㄉ" with bopomofo "ㄇ ㄋ" in the editor, we still cannot type this phrase by the bopomofo we added. And the editor should avoid to adding all these invaild phrases not just this case.

[harryoooooooooo]

but actually I can type this phrase after I map "ㄅㄉ" to "ㄇ ㄋ".