Enable emphasis in multi-byte sentences when no_intra_emphasis is set

Currently the `:no_intra_emphasis` extension disables emphasis in multi-byte sentences.

That is not desirable, because words are not separated by spaces in multi-byte language like Chinese or Japanese.

For example, given the following sentence (in Chinese):

    中文內無空白的**粗體**也應加粗。

should render like this:

中文內無空白的**粗體**也應加粗。

The two characters surrounded by asterisks should be bold. It renders correctly on Github but not on redcarpet.

This pull request fixes this issue by adding a `_isalnum` function checking if the given character is an ASCII alphanumeric character. (The code is borrowed from github-markdown.)

I hope this issue can get fixed so that multi-byte language users are freed from adding extra spaces into sentences. Thanks! :smiley: