Add ability to set environment variables for an execution

[khamer]

Working through some use cases for the buddy-cli, and the ability to set environment variables for a specific execution seems really handy, and already exists within the API.

This PR adds that functionality, following this format:

$ buddy-cli -w WORKSPACE -p PROJECT pl run PIPELINE -e key1=val -e key2=val2

I added it to the README as well. Let me know what you think and if there's other changes you'd like to see.

[khamer]

I missed the 'Pass arguments' action - there might be worth I could do on this PR to better integrate it with that action.

[akondas]

Hey @khamer, thanks for contribution, can you also add test for this? One more thing, -e key1=val -e key2=val2 this can also be shown in readme as example :wink: