Support re-evaluation of dotenv files at runtime.

[trulede]

Description

Enable the re-evaluation of dotenv files (global/task) if the file changes. Supports this method:

version: '3'
dotenv:
  - cred.env
tasks:
  login:
    run: always
    cmds:
      - echo "FOO=foo" > cred.env
  doit:
    deps:
      - login
    cmds:
      - echo foo=$FOO               # foo=foo
      - echo foo={{.FOO}}           # foo=foo
      - echo "FOO=bar" > cred.env
      - echo foo=$FOO               # foo=bar
      - echo foo={{.FOO}}           # foo=bar

General refactoring tasks:

• templater saved as member of Task (compiledTask)
• cmd's are expanded just before being run (i.e. in runCommand)
• keep track of envar files, rebuild templater if changed
• create function to cleanly generate a templater object (decouple from existing code)

Possibly, the vars code can keep track of dotenv files. Create a type to do that efficiently.