refactoring CLI commands and documenting

[maxlath]

[this is a the same as #58 but with a different branch to be able to customize master]

hi! I was struggling to use the CLI, so I dug into the code and went into refactoring berserk mode: it might be a lot of changes, so feel welcome to merge or cherry-pick whatever make sense to you :)

Summary:

• documented CLI commands
• renamed the --createReadStream command into --all to distinct it from --keys and --values streams
• Added a --line mode for --keys
• Added a --length command
• Added a --map command
• Fixed the --batch option and let it take operations from a file
• made --all and --batch be fit to be used as import/export tools
• Merged CLI stream commands code to mutualize logic

[Edit: those changes are now part of lev2]