Implement persistant storing of checksums

[danylkaaa]

We need to store checksum persistent, if we want to be able to start working on #32

[kindermax]

My view of functionality:

0. add new option to command in yaml called persist_checksum - it is a boolean
1. create a folder called .lets in the project directory (where lets.yaml exists)
2. in .lets folder there will be a checksum folder
3. each checksum will be stored into a separate file called {command}_{checksum_name_if_exists}
4. when a command runs for the first time we write down new checksum into a file
5. when a command runs the second time we reading an old checksum from a file, calculating the new checksum, compare them and expose new environ variable LETS_CHECKSUM_{NAME_OF_CHEKSUM_IF_EXISTS}_CHANGED - a boolean
6. the most important thing - we replacing old checksum with a new checksum only if a command exits with status 0. That is a command using a checksum persist must be as deterministic as possible - for example, docker build or npm install. That gives a guarantee that we replacing a checksum only if we make sure our command is completed successfully.

[danylkaaa]

Maybe we should creeate GitHub project board? And use it for releases?

[kindermax]

#43
https://github.com/lets-cli/lets#persist_checksum
https://github.com/lets-cli/lets/releases/tag/v0.0.20