RFC: Add session option

[laburnumT]

Adds a session option to codeblocks so previous results can be used.

To check that a command is done running, we wait for an eof_token to appear. This allows us to not cut off commands with output early, but also doesn't rely on using sleep.

[laburnumT]

This is an RFC for adding session support to vim-medieval. It's currently quite
hacky, because repl's aren't always accessed the same way. But that's why it's a
first draft.

It currently has some checks in there for python (because that's what I use in
markdown for the most part).

This should probably be integrated into g:medieval_langs, or a new config
variable, but I just wanted to know if there is any interest in functionality
like this before I do that work.

[laburnumT]

The main problem this is trying to address is where something is either costly
to run or not idempotent. As a rather stupid example:

<!-- session: example -->
```python
import random
import time

time.sleep(5)
x = 42
```

<!-- session: example -->
```python
print(random.randint(1, x))
```

Using require here would make the print block always take 5 seconds, whereas
with a session it would run instantly (as long as the first code block has been
run at least once)