Allow global environment variables

[nashidau]

Currently each task has it's own environment variable. It would be nice to be able to environment variables which are shared with all the tasks set under the ## Tasks header but before each target.

Inputs would be good too, but I fear it may make the syntax trickier.

Tasks

environment: SERVER=http://localhost:8545

getrandom

curl -X POST $SERVER \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "method": "getRandomInt",
    "id": '$$'
  }'