Ensure code spans put in emphasis work correctly

Hello,

When putting code span into simple or double emphasis, we've got an unexpected output. This commit fix this problem simply removing an ignore statement about back ticks in the `find_emph_char` function.

This pull request fixes #135 and #190.

All tests are green on my machine (with 2.0.0-p0) ; hope the Travis build will be too!

Have a nice day.