[Proposal] Embedding other markup without adding more grammar

_Before I raise any PR, I wanted to check if this is feasible._

I'm writing a story and I'd like to include Markdown as the markup language. There's a bit of overlap between markdown & Ink and a few characters should be escaped. Example:

```
-> Menu

=== Menu ===

\**Select one**
_Select one_
`Code`

listitem:
1. item1

\- item1
\- item2

\--- // new line
```

There are a few more edge cases (escaping URLs, for examples). Searching in the issues for this repo reveals:

- Issues with the `*`. https://github.com/inkle/ink/issues/275
- URLs. https://github.com/inkle/ink/issues/153#issuecomment-617548445
- Hash symbols https://github.com/inkle/ink/issues/560
- Square brackets https://github.com/inkle/ink/pull/385

Most of the issue can be solved with escaping — e.g. `\*`. But if the text contains several of those, it's really hard to write a story and escape all the characters as well.

I looked in the documentation for an instruction to escape a block of text. Example:

```
-> Menu

=== Menu ===

{raw
**Select one**
_Select one_
`Code`

listitem:
1. item1

- item1
- item2

--- // new line
}
```

But I realised that Ink doesn't offer such functionality.

I tried to look for other workarounds and realised that I could escape the text with a function:

```
EXTERNAL md_bold(string)

-> Menu

=== Menu ===

{md_bold("Hello")} World!

-> END
```

The issue is that I can't have arguments that span multiple lines:

```
EXTERNAL MARKDOWN(string)

-> Menu

=== Menu ===

{MARKDOWN("Hello
new line")} World!

-> END
```

So I was wondering if it is possible (and acceptable) to contribute a feature to:
- Either extend the language with a new word to escape blocks of text.
- Extend function arguments to span multiple lines.

Thanks,
Dan

P.S.: Both features enable a (convenient? annoying?) side effect. I can embed any other markup language in Ink and I can write my own extensions without changing the compiler.