Add a Dream.livereload middleware

[tmattio]

The code is adapted from dream-livereload, itself adapted from the w-live-reload example.
We update the w-live-reload to use the newly introduced middleware.

[aantron]

Thanks!

Note: suggested here, just so the issue has a backlink.

[aantron]

@tmattio, I think we should remove the unit argument. The unit argument makes Dream.livereload () compose better with Dream.no_middleware inside an if-expression. But we already have middlewares that may be used in development but that don't have the unit argument and therefore don't compose well inside if, for example, Dream.memory_sessions:

dream/src/dream.mli

Line 1573 in 6e666d4

val memory_sessions : ?lifetime:float -> middleware

Also, this approach is not future-proof, because any middleware could gain an optional argument in the future, and we won't be able to add unit arguments to them without breaking users. This does mean that adding any optionals to any middlewares will break them if they are used inside if-expressions, but that's just a general flaw of optional arguments in OCaml.

We will just add an Dream.if_ combinator later if needed. For example:

let if_ : bool -> then_:Dream.middleware -> Dream.middleware =
    fun condition ~then_ ->
  if condition then
    then_ 
  else
    Dream.no_middleware

It's also possible for the user to work around this using a type constraint, Dream.livereload : middleware, although, of course, this is awkward.

Separately, are the ?script and ?path options really needed? We can probably get by for now without them. I'd rather add them later on request, than support them when there is no evidence of them being used (I mean, I don't have any -- you may well have!).

[aantron]

@tmattio, Thanks for the PR! We left you a bunch of comments, but in case you are pressed for time, etc., I'm ready to take over the PR, push in a bunch of small changes and squash it down, crediting you :)

[tmattio]

Thanks for the review @aantron (et al. 😉)!

I'm happy to continue with the PR and address your comments but probably won't have time before next week - if you have time before that for sure feel free to take over, otherwise, I'll push patches as soon as possible

[aantron]

@tmattio, I'm taking over this PR to merge it in!

[aantron]

Thank you!