Cascading cheat sheets

With our "cascading cheat sheets" mode (it will be described in README.md soon), you can create a hierarchy of cheat sheets servers, where each server delivers local cheat sheets + if a requested cheat sheet is not found, the query is forwarded to the upstream server (and so on).

```
cheat.sh
^
|
cheat.my
^
|
clients
```

For the user it looks fully transparent, it uses the same clients as always. The only difference is that the clients should be configured the correct server (is some clients are used; of course, if you use nothing but curl, you change nothing but the name of the server in the query):

```
curl cheat.my/secret
```

In this query, you ask the `cheat.my` server for the `secret` cheat sheet.

The main advantage of this mode is that you can have your own (private or maybe public but relevant only to you and to your team) cheat sheets, and at the same time, you can use the global cheat sheets to, without changing anything.

This issue is related to #5, #14, #97

To do:

- [ ] server side implementation
- [ ] cheat.sh docker container (almost implemented by @bglopez)
- [ ] documentation (README.md)
- [X] cht.sh client custom server support
- [X] cheat.sh-vim custom server support (@dbeniamine)
- [X] vscode-snippet custom server support (@mre)
- [X] cheat-sh.el custom server support (@davep)
- [ ] cheat.sh-sublime-plugin custom server support (@gauravk-in)
- [X] windows client custom server support (@tpanj )