Bug fix: Underscore emphasis missing when followed by punctuation

An issue was identified in Extra mode (#25) where underscore emphasis was not being converted to HTML correctly if followed by punctuation.

#### Example:
`_internal_,` would be incorrectly rendered to `_internal_,`, when the expected output is `<em>internal</em>,`.

I added a test for this and implemented a fix for both the C# (aeafdb7bcaa3b6750af1f2d001be03a1118ae747) and JavaScript (31ac931b97db2e50d9ebc054e0e8c6aba9711122) projects.

Thank you for your work with MarkdownDeep, it is an excellent library. Please feel free to contact me if you have any questions or feedback.

Best regards,

Evan

> *Note:* This pull request has been modified and re-requested with a reduced impact to the overall code base. I hope it will be considered for merge as soon as possible.