Include POP action type as a public action type

[pmeskers]

This is a small PR which adds the POP (ROUTER_POP) action type to the public exports in src/index.js. Of all the types defined in src/types.js, this is the only one that was not included in the list of "public action types".

From what I can tell, it's quite useful since it's the only action which fires for both router induced back behavior (e.g., goBack()) and for browser induced back button (e.g., the browser's back button).

[codecov]

Codecov Report

Merging #218 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #218   +/-   ##
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  Coverage   98.41%   98.41%           
=======================================
  Files          19       19           
  Lines         253      253           
=======================================
  Hits          249      249           
  Misses          4        4

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