Allow GFM autocomplete to be trigger without the preceding space

## What does this MR do?

Gives the ability to GFM autocomplete to be trigger even if there is no preceding space.

I've taken the regex from the at.js plugin & tweaked it to allow the leading character to be a special character.

## What are the relevant issue numbers?

Closes #19975 

## Screenshots (if relevant)

![Screen_Shot_2016-07-21_at_14.41.34](/uploads/19684ba286baeedb754e7457945480a8/Screen_Shot_2016-07-21_at_14.41.34.png)![Screen_Shot_2016-07-21_at_14.41.40](/uploads/a77349bce599ae93b4bcddd355087f5c/Screen_Shot_2016-07-21_at_14.41.40.png)![Screen_Shot_2016-07-21_at_14.41.46](/uploads/c35df17b678b24b73c94b181f0784188/Screen_Shot_2016-07-21_at_14.41.46.png)

See merge request !5395