Allow plugin to assign priority to result block

[linonetwo]

#269 don't solve the sorting problem.

Expected Behavior

I want fallback block's google search on top of browser history block, but below the translation block.

Actual Behavior

When I want to search google, but unrelated fuzzy searched browser history result pop on the top.

Steps to reproduce the behavior

Versions

• Zazu: 0.6.0

Maybe we can allow plugin developer assign a priority value, and set fallback plugin's priority to 0, so some developers can set their priority to -1 to be on the bottom, and to 100 to be on the top.

And user can set variables in the config to override this plugin priority.