Initial cheat repo support (#258)

This PR makes navi stop bundling `.cheat` files and makes it able to download from repos on GitHub, using a standardized config directory.

- it fixes #233 and #237
- it makes #52, #256 and #257 much easier to implement
- it makes #40 harder to implement
- it's an alternate solution to #140 
- it's influenced by #238 and #254.

This PR ended up being much bigger than I would like so if you could review only this description it would be already very nice!

When navi is started for the first time, a welcome cheatsheet is shown:
![Screenshot at 2020-03-15 10-20-04](https://user-images.githubusercontent.com/3226564/76702240-19fffd80-66a7-11ea-884f-97c565bc1ead.png)

If the user selects the first option, the user is prompted to download `.cheat`s from https://github.com/denisidoro/cheats: 
![Screenshot at 2020-03-15 10-20-35](https://user-images.githubusercontent.com/3226564/76702239-19fffd80-66a7-11ea-8f69-324f669b1e01.png)
![Screenshot at 2020-03-15 10-22-59](https://user-images.githubusercontent.com/3226564/76702236-18363a00-66a7-11ea-8ff4-53b497f85888.png)

The config folder is populated:
![Screenshot at 2020-03-15 10-21-11](https://user-images.githubusercontent.com/3226564/76702238-19676700-66a7-11ea-8367-3e7b5733f2b4.png)

When run again, cheats are available:
![Screenshot at 2020-03-15 10-21-34](https://user-images.githubusercontent.com/3226564/76702237-19676700-66a7-11ea-9c2a-d8829340f3e9.png)

### Breaking changes

In order to make navi stop bundling shell widgets as well, a breaking change has been introduced: `source $(navi widget bash)` won't work anymore. It should be `source <(navi widget bash)` now. Any ideas on how to make this transition more graceful?

scripts/action

@@ -33,8 +33,6 @@ release() {
    chmod +x "$bin_path"
    mkdir -p "$TAR_DIR" 2> /dev/null || true
 
-   cp -r "${NAVI_HOME}/cheats" "$TAR_DIR"
-   cp -r "${NAVI_HOME}/shell" "$TAR_DIR"
    cp "$bin_path" "$TAR_DIR"
 
    cd "${NAVI_HOME}/target/tar"