Add argument handling code

* It is now possible to add arguments in the URL bar of the browser
* If they are any of the arguments that may be accepted:
  - They are acted upon by the JS script that powers the page
  - And are passed on to the dataparsing PHP script handling the backend
Move files to prepare for mod parsing

* 0ad-specific content is now in ./mods/0ad
Amend code to point at new location for core 0ad content

* Also defaults to core 0ad content if no mod is specified in the arguments passed to the techtree
Add a few community mods as submodules to play with

* The following mods have been added:
   - "Aristea" (aristeia)
      -- and its dependency "convert_attack"
   - "Rise of the East" (han_china)
   - "Millennium AD" (millenniumad)
   - "Lusitanians" (lusitanians)
Add ability to use mods

* By using `?mod=[mod_name]` in the URL bar, it is now possible to view the techtree of a custom mod
* The `mod.json` file in the requested mod will be parsed to determine the mod's dependencies
   - a mod's dependencies are also checked for their own dependencies, and so on
   - versioning info (ie `0ad 0.0.16`) is ignored for now
Add ability to select mod from dropdown

* The folder of mods is now scanned for valid mods, and a new select UI element is populated from this
* This means a user can now select a mod from this drop-down, rather than having to remember what its called and type it in
Permit more than one mod to be enabled

* Previously the only way to have two (or more) mods to be loaded simultaneously is to have one as the dependency of the other
* Now, it's possible to enter more than one `mod` arg into the address bar and have them all load (if valid)
Permit users to be able to select multiple mods from UI

* So a user doesn't have to enter them manually into the address bar
Move CSS styles to their own file

* Tidies up the header of the index.php file
Filter mods so 'functionality'-only mods are hidden

* This type of mod doesn't include any new civ or tech data
* Although they need to exist in the filesystem so as to satisfy dependencies, they shouldn't be choosable from the UI
* Change also swaps round mod.name and mod.label in their use in the UI