Add ability to skip tasks,if checksum for this traks was not changed

[danylkaaa]

Sometimes I want to skip execution of some tasks, if command's checksum from last run is not changed.
Example: before each npm run smth I want to run npm i. But if package.json and package.lock is not changed, I do not need to run npm i

So, in lets.yaml I want to past smth like this and as a result, omit running task

New lets.yaml

install_deps:
  checksum:
    deps: &deps
      - package.json    
      - package-lock.json
  only:
    checksum:
      <<: *deps
  cmd: npm i

run-smth:
  depends: [install_deps]
  cmd: npm run smth

As you can see, in lets.yaml I added in cmd install_deps property only. It will allow to execute install_deps.cmd only if checksum for this cmd changed.

So, what we need to implement:

• Persistant saving of checksums into file
• Reading saved checksum for command
• Updating of after cmd successful execution
• Skip command invocation, if new and saved checksum are equal

[danylkaaa]

Later we can add some other rules for only property in cmd, like Git branch name and etc ( analogy to gitlab-ci only)

[kindermax]

I think we have to implement checksum persistent first (probably in a separate issue) and then implement only functionality

[danylkaaa]

Makes sense

[kindermax]

Another use case. When using depends, sometimes we want to skip some of the commands in it.

There are a couple of ways to do that.

1. Persistent - store settings in .lets (not implenented)
2. Dynamic - for example lets -E PULL=false run
   If commands have if or only directive, we can check PULL in it and skip the command (not implemented)