add pathPrefix configuration

[foxnewsnetwork]

Description

We want to add the ability to customize pathPrefix for github pages deploys

Related Issues

#281

Changes

• add pathPrefix option to config.js
• pass pathPrefix to gatsby-config
• update readme with github deploy instructions

Notes

Despite what I said in #281, apparently it's super easy to add pathPrefix capabilities without having to modify the js code at all

[codecov]

Codecov Report

Merging #283 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #283   +/-   ##
=======================================
  Coverage   97.22%   97.22%           
=======================================
  Files          35       35           
  Lines         144      144           
  Branches        8        8           
=======================================
  Hits          140      140           
  Misses          4        4

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[alxshelepenok]

Nice. Thanks for the PR!