Add colons to the alternative text as well

From downstream package frissdiegurke/nodebb-plugin-emoji-extended#5:

We're experiencing problems when users copy and paste emojified text. The emoji embedded does not contain the colons as expected, so when *re-pasted* back into a textarea that will get emojified, the emoji itself is missed.

Raw: `Some text with emoji :ice_cream: :hatched_chick:`
Copied into clipboard: `Some text with emoji ice_cream hatched_chick`
Pasted & re-emojified: `Some text with emoji ice_cream hatched_chick`