[Rd] Exposing native httpd server

[Jiří Moravec]

R has a native HTTP server that is used for serving R help pages 
interactively, at least on the loopback device (127.0.0.1)

But all of the working are internal, not exposed to user and not documented.
This is quite shame since the server seems to be fully capable of 
handling basic tasks,
be it serving static websites or even interactively processing queries.

This was previously noticed by Jeffry Horner, the author of the Rook 
package.
I am just a guy who found it interesting.

The basic working is as follows:
User needs to either overwrite the internal `tools:::httpd` function or 
add their hook into the internal environment tools:::.httpd.handlers.env.

In the former case, the user will be of a full control of the server, in 
the later case, the `app` will be hooked to `/custom/app` instead.
All that is needed then is to run the interactive help that starts the 
webserver.

Based on the breadcrumbs left on the way, I was able to write a server 
that emulates much more complex `servr` package that I have previously 
used to test locally my blog.

https://gist.github.com/J-Moravec/497d71f4a4b7a204235d093b3fa69cc3

You can see that I am forced to do some illegal procedures:
  * tools:::httpd needs to be replaced
  * the server doesn't have knowledge of a directory so setwd needs to 
be set
  * the function must not end, otherwise the directory is changed during 
the server lifetime (and depends on the current working directory)

I would like to suggest and probe for willingness to expose the native 
http server.
This would include:

* de-hardcoding the server so that we can register other functions not 
just httpd
* exporting many functions and renaming them (such as mime_type)
* writing better interfaces, `startDynamicHelp` is kind of hard to work 
with, something like httpd_start(dir, fun, port), httpd_stop(port) and 
httpd_status(port) would be much cleaner.

I would like to say that I have no idea what I am doing, I don't 
understand webtech or the internal implementation, so if there are 
reasons why this isn't a great idea...

I am happy to make a PR for the R part. 
https://github.com/wch/r-source/blob/trunk/src/library/tools/R/dynamicHelp.R
The C part with the R's C internals look to me like a black magic and I 
don't feel confident enough. 
https://github.com/wch/r-source/blob/trunk/src/modules/internet/Rhttpd.c

See this old stackoverflow answer, where someone was looking for `python 
-m SimpleHTTPServer 8080`

https://stackoverflow.com/q/12636764/4868692