Allow specifying list of environment variables.

[swhite24]

Thanks for the great project. I've recently come across a case where I have many environment variables I'd like to provide to the binary and listing them all inline with the full_bin key was becoming a bit unwieldy.

This PR adds a new key, env, which will accept a table of inputs to prepend to the bin command as environment variables.

For example, this config (truncated):

# air.conf
[build]
bin = "tmp/main"
env.PORT = "3000"
env.ENV = "production"

Will run the following:

PORT=3000 ENV=production tmp/main

[udaya2899]

Wouldn't it be better to ask the user to have the list of env files as export commands in a separate file called dev.env and source the file before building, with bash it'd be source dev.env