Allow configuration of where the RPC content script is read from via rpcContentScriptDir

[nessup]

This allows the RPC content script to be arbitrary read from any directory, rather than the default __dirname + '../rpc/rpc.js. This is important in situations where node_modules may be inaccessible in certain situations, such as in production builds.

[googlebot]

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[paulirish]

@nessup would it be possible to use require.resolve() instead of introducing configuration?