Change prefix logic to check for following vowel

[dchiller]

This PR:

• Adds co to the list of prefixes
• Adds a test word coegerunt (co-e-ge-runt) that has this prefix
• Changes the prefix logic so that a prefix only requires special handling if it is followed by a vowel. The reasons for this are outlined in the changes this PR introduces to the README. In short, if the prefix is not followed by a vowel, then the usual consonant-placement rules apply and no special handling is required.

Closes #46.