Added router to support built-in PHP web server

[ftrrtf]

I was surprised when I saw that there is no way to run grav on the built-in server, which is ideal for development, 'cos grav doesn't need a database, etc. So I'm a little researched this issue and found a solution.
Used for the past several months for me, no any problems noticed, everything works as well.

How to run:

php -S localhost:8000 router.php

Related issues:
getgrav/grav-plugin-admin#2
getgrav/grav-plugin-admin#3

[flaviocopes]

The problem is not that Grav does not run on the built-in webserver. It does, and it works fine.

But Admin, and some other plugins, do not because of getgrav/grav-plugin-admin#3 (comment).

We therefore added a check inside Admin to prevent it to run on the built-in webserver.

BUT your solution makes it work, very nice! Just not sure if / how to handle this, as people do not always read the documentation before firing the built-in webserver, so we'll get those problems back when they run php -S localhost:8000. Maybe we could have this documented somewhere, we point out to this in the error we give, and we add a setting to unlock the built-in webserver in the admin config yaml.

[rhukster]

Thanks for this PR @ftrrtf, i've made a few minor modifications to ensure this can't be run without the router. Please check my PR: #1219

If it's all good for you we can merge that one.

[ftrrtf]

@rhukster 👍

[rhukster]

Closing this one.. please move discussions to #1219