Don't insert emoji in the middle of a word

Here's a patch that solves a bug I was seeing in my use of emojify.js:

https://github.com/qq99/echoplexus/issues/235 the bug in question

When emojifying a plain old string, sometimes it was inserting emoji mid-word.  This is especially likely in URLs that use hyphens (the one in question was `https://hacks.mozilla.org/2014/06/introducing-the-web-audio-editor-in-firefox-developer-tools/` which has `x-d` in it that got converted)

Making this change doesn't break any tests, as far as I can see