Add pre-description and post-description for help output.

[Gökmen Görgen]

My planned help output is like this one:

$ dosh help

dosh 0.1.0
shell-independent command manager

Tasks:
  > setup_my_os        place my config files.
  > install_cli_apps   install my favorite apps.
  > sync_repos         do not forget to push your changes before working on a different
                       laptop.
                       - repo_path: str, optional

If you are new to dosh, consider running `dosh --help` to learn using dosh.

This help output has three parts:

• the first part includes the app name, version, description, etc.
• tasks that show the available subcommand and its parameters.
• the last line is about the --help parameter.

I want to allow the clients to edit the first and the last parts.

[Gökmen Görgen]

Created a new PR: https://git.gokmengorgen.net/goedev/dosh/pulls/3

[Gökmen Görgen]

Merged.