feat: add source field to each cheatpath to describe cheatsheet's source, add command option to print current config path

[Lonenso]

an alternative way to resolve the problem #658 mentioned.

a glance of sample config yaml

  - name: community
    path: /home/lone/cheatsheets/community
    source: https://github.com/cheat/cheatsheets.git    # add this key.
    tags: [ community ]
    readonly: true

also add an command option -C/--config to show config path

> cheat -C
config: /home/lone/.cheat.yml

PS: sorry about the low version pandoc, it caused some unwilling changes.