Allow rendering emojis when not surrounded by spaces

[iksaku]

GFM allows rendering emojis without requiring the markers to be surrounded by spaces, even so, you can render multiple emojis continuously.

The current implementation of this extension doesn't allow emojis to be rendered if they are not surrounded by spaces.

Look at the following table for comparison:

MD String	CachetHQ's Render	Github Render
:+1:123		👍123
:+1::+1:		👍👍
¡Viva Mexico :mexico:!		¡Viva Mexico 🇲🇽!

Hope you agree to merge this 👐.