New option 'count' to display arbitrary number of results, rather than always 10

[FosseWay]

Rather than always displaying the top 10 most-visited pages, it may be useful for some editors/admins to specify a custom limit (e.g. Top 5 or Top 20) on a particular instance of {{top}}, which could be specified with a new option count= (e.g. {{top|count=5}} or {{top|count=20}}).

There should be sensible limits: not fewer than 1 result, and not more than 20 (which is already a hard limit in the code as-is).

Basic valid-value enforcement of the value of count would be useful:

Count value is...	Value enforced
Non-numeric	10
<1	1
>20	20