Feature request: please add custom style syntax

First of all thank you for this great crate! I am using `termimad` to implement a CLI tool which should output text in different colors, not related to bold, italic, code, or strikeout styles. In other words, once I have personalized my bold / italic / code / strikeout styles, then I also need to apply different colors to both stylized and normal text.
It would be great to add the possibility to define new custom syntax. For instance, with reference to the following [example](https://docs.rs/termimad/0.21.0/termimad/#the-skin):
```rust
skin.strikeout = CompoundStyle::new(Some(Red), None, Bold.into());
```
since I need to keep the `strikeout` style for some text, and also apply red bold style to some other text, it would be great to be able to write something like:
```rust
skin.custom_style("rb") = CompoundStyle::new(Some(Red), None, Bold.into());
```
and then apply it for instance as:
```rust
termimad::print_inline("#rb#red bold text!#rb#");
```