[mono-api-html] Allow ignoring parameter names, virtual.

I've been trying to review API diffs in Xamarin.Android between API
levels, but the diffs are "noisy" because Google will change parameter
names at-will, or will change a non-virtual method into a virtual
method, or a virtual method into an override, all of which are
acceptable changes but individually generate a Removed line in
mono-api-html output, and manually removing the *hundreds* of changes
is error-prone.

Add two new option flags to mono-api-html:

  --ignore-changes-parameter-names:
      Ignore changes in constructor and method parameter names.

      If parameter names change between <reference.xml> and
      <assembly.xml>, don't report a change for the constructor or
      method.

  --ignore-changes-virtual:
      Ignore changes in "virtual-ness".

      If a method or property changes from non-virtual to virtual,
      non-virtual to override, or virtual to override, don't report a
      change for the member.