Adding ability to process js, graphql, and other arbitrary file types (e.g. .gql)

[mergebandit]

extension property / argument now supports multiple file types.

This allows users to execute persistgraphql in environments that utilize the gql tagged template literal, .graphql files, .gql files, and any arbitrary extension that consumers have defined for graphql files.

Specifically, the persistgraphql-webpack-plugin defines a loader for .graphql|.gql files, which was my original aim to support.

Fixes #50

[apollo-cla]

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[Poincare]

@mergebandit Thanks so much for the PR! I mentioned a couple of changes above that should be pretty easy to implement. I think the concept is fine otherwise.

[mergebandit]

@Poincare - left some responses above.

[Poincare]

@mergebandit Yup saw those - will respond to each in about 1-2 days.

[mergebandit]

Bump.

[mergebandit]

Also - I'll need to bump the minor version in my next commit (reminder to myself)

[mergebandit]

@Poincare I (and others on my team) would be more than happy to assist in ongoing maintenance and feature development for this plugin.

[mergebandit]

Is this ever going to be addressed or should I just maintain a fork moving forward?