A way to enforce & check required variables

[benc-uk]

There have been several requests (#889, #714, #495) for a way to enforce or check that variables are set before running a task. There have been several solutions proposed, but most of them require some fairly obscure use of template functions, bash tricks, or are a little verbose/clumsy.

Being able to define what vars are required before running a task is pretty important IMO, esp for developer experience. It prevents that "oh shoot I forgot that one var in my .env file" problem, guides people towards the answer, rather than letting some command or script depending on that variable fail in cryptic & possibly dangerous ways. Sometimes when running a deployment or release a missing variable might not be obvious until a significant time has elapsed.

Proposal

A new field called requires is added to the task struct, this field is an array of strings. These strings should be variable names, and these variables are checked for existence before the task is run. If one or more is found to be missing, the task will not run and an error describing the missing variable(s) is returned.

Simple example

version: '3'

tasks:
  docker-build:
    cmds:
      - 'docker build . -t {{.IMAGE_NAME}}:{{.IMAGE_TAG}}'

    # Make sure these variables are set before running
    requires: [IMAGE_NAME, IMAGE_TAG]

For discussion:

• Is requires a good name? It's short and elegant, but maybe implies some other dependency? I considered requiredVars but the camel case made me cringe a bit.
• I considered the idea of allowing users to specify the message in a failed check, but this significantly distracts from the elegance of a simple array of strings

I believe this important enough to be a "1st class" feature of task, so I shall be submitting a PR shortly to implement this feature