Prefix search shortcuts with the ':' character

[capouch]

Did you search for similar issues before submitting this one?
Yes

Describe the issue you encountered:
Went to search for any number of terms beginning with the letter 'a' followed by a space. All of them search Amazon instead of the web.

Expected behavior:
Web search.

• Platform (Win7, 8, 10? macOS? Linux distro?):
  Linux, Slackware
• Brave Version:
  0.12.1
• Steps to reproduce:
  1. Spawn new tab
  2. Search for "A search like this will go to amazon."
  3. Wince
• Screenshot if needed:
  

Browser has been launched for many hours and lots of tabs are open. Maybe others will not be able to reproduce.

• Any related issues:

[bbondy]

we were just having a discussion about this last night. I think we'll do :a as the different engine prefix.