modified loadGatsbyConfig to support TS

[graydonpleasants]

Summary

Modified the plugin to also look for a gatsby-config.ts file, something now supported by Gatsby.

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[pieh]

Does this actually work? From reading this it would try to require() a typescript file which wouldn't work in default Node.js environment unless some kind of "transpile TS on the fly hack" was used (for example ts-node/register)