Support local environment variables

[fljdin]

Inspired by named connections for psql tasks, it should be nice to defined dedicated variables per task

---
environments:
  - name: remote
    variables:
      PGHOST: remote
      PGDATABASE: db
  - name: default
    variables:
      PGDATABASE: postgresql://localhost/?service=local

tasks:
  - id: 1
    command: vacuumdb
    env: remote
  - id: 2
    command: vacuumdb --analyze-only
    variables:
      PGHOST: localhost
      PGUSER: postgres
  - id: 3
    type: psql
    env: remote
    loaded:
      from: psql
      variables:
        PGHOST: remote
      command: |
        SELECT format('sleep .0%s', i) FROM generate_series(1, 2) AS i