Context: dotnet/android#1078

xamarin-android uses `mono-api-html` and `mono-api-info` in order to
perform API comparisons; see e.g. 4e45904.

A recent problem has arisen regarding "inter-API-level diffs": there
are many API changes which we consider to be acceptable, and thus
would like to ignore. Unfortunately, `mono-api-html` doesn't support
ignoring many of these constructs, e.g. base class changes:

	<h3>Type Changed: Android.Preferences.CheckBoxPreference</h3>
	Modified base type: <span class='removed removed-inline removed-breaking-inline'>Android.Preferences.Preference</span> <span class='added '>Android.Preferences.TwoStatePreference</span>

and property type changes:

	<h3>Type Changed: Android.Views.InputEvent</h3>
	<p>Modified properties:</p>
	<pre>
	<div data-is-breaking>	public <span class='added added-breaking-inline'>abstract</span> int DeviceId { get; }
	</div><div data-is-breaking>	public <span class='added added-breaking-inline'>abstract</span> InputSourceType Source { get; }

Overhaul the `mono-api-html` "ignore" infrastructure:

  * Introduce ignore "scoping". Previously, the ignore options would
    only match the *member* text against any provided regular
    expression. Thus, `mono-api-html -r ToString` would ignore *all*
    members containing `ToString`. Furthermore, there was no way to
    restrict the ignore to a particular type. Expand the regex
    semantics so that the declaring type is used as a "prefix" for the
    to-be-matched text. For example, if a class `Example` is removed,
    `mono-api-html` will now match the following value against any
    `-r` regular expressions:

        Example: Removed type

  * Add a `mono-api-html -v` option, to control output verbosity.
    When specified, `mono-api-html` will print out available `-r`/etc.
    option values:

        $ mono-api-html expected.xml new.xml -v
        Possible -a value: Android.Resource: Added fields: public static const int AccessibilityEventTypes;
        Possible -r value: Android.Preferences.CheckBoxPreference: Modified base type: 'Android.Preferences.Preference' to 'Android.Preferences.TwoStatePreference'
        Possible -r value: Android.Views.InputEvent: Modified properties: public int DeviceId { get; }
        Possible -n value: Android.Views.UnavailableException: Added type
        ...

Additionally, allow `mono-api-html` to take a
`Mono.Options.ResponseFileSource`, which allows `@response-files` to
be used as command-line options.