Include environment variables from config

[kchung]

On a production lambada, you can access your netlify.toml environment variables, but in this package the same isn't available during development, this PR adds the variables from the config on the the process.env just like how production works.

• Include netlify.toml environment variables

[leonardodino]

awesome addition! :D

just wish i could access my "private" environment variables, the one not checked into the repo, the ones set on the dashboard, maybe expose all the build process.env to the child process? or some way to configure it? I know the ones on the toml overwrite the dashboard ones..

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update: moved to using DefinePlugin on the the custom webpack, config, solves it for me.