Support searches like the following:
* score:<0,>100 (equivalent to `score:<0 or score:>100`)
* score:5,10..15,>20 (equivalent to `score:5 or score:10..15 or score:>20`)
* id:5,10..15,20..25,30 (equivalent to `id:5 or id:10..15 or id:20..25 or id:30`)
This also works inside the `search[id]` URL parameter, and inside other numeric
URL search parameters.
Factor out the code that parses metatag values (e.g. `score:>5`) and
search URL params (e.g. `search[score]=>5`) into a RangeParser class.
Also fix a bug where, if the `search[order]=custom` param was used
without a `search[id]` param, an exception would be raised. Fix another
bug where if an invalid `search[id]` was provided, then the custom order
would be ignored and the search would be returned with the default order
instead. Now if you use `search[order]=custom` without a valid
`search[id]` param, the search will return no results.
