Allow UI to inject api keys to the build.

I am using sentry.io for crash reporting in my hybrid app.But since the app is also opensource i need to hide my keys from the code.
As a workaround, i have resorted to adding the config.js file in .gitinore, but this is not an ideal solution because I have to build the binary beforehand and commit it with the code. @danrabbit has pointed out that there are security issues with this approach ( verifying code is hard ).

This allows me to build hybrid apps with vala and nodejs
https://github.com/harisvsulaiman/pushy/issues/13

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Start by reviewing the repository's build and configuration entry points, then compare the requested behavior with the referenced pushy issue and the existing config.js workaround. Define how the UI would supply API keys without committing them and how a build can verify the resulting binary; the issue is done when that flow is specified and tested.

Written by the indexing model from the issue text.