Add support for 'decimalOnly' encoding option

I had a need to only encode with decimal entities, so I went ahead and modified the encode method as you suggested to take an alternate parameter to toggle this on/off. I also added some new tests, updated the readme, and bumped the version number.

Note, in order to simplify the code, I also pulled in the other pending pull request on your repo to simplify the semicolon handling -- so this pull request includes both sets of changes.

If you find this to be a useful addition to your library, I'd love it if you could push an updated version to npm so I can use the 'official' release!

Let me know if you have any questions or concerns.