Fix for calling mention_link_filter with only one argument

[benbalter]

Currently, the 4th argument for HTML::Pipeline::MentionFilter#mention_link_filter, username_pattern, is required, even though args 2 and 3 are optional.

I believe this is a regression introduced in #157.

I copied the default argument format of mentioned_logins_in to default the final argument to UsernamePattern and added a test (the mention_link_filter had no tests previously AFAIK).

Before:

> filter = HTML::Pipeline::MentionFilter.new nil
=> #<HTML::Pipeline::MentionFilter:0x007fc70aa759d0 @doc=nil, @html=nil, @context={}, @result={}>
> filter.mention_link_filter "@hubot"
ArgumentError: wrong number of arguments (1 for 2..4)
    from /usr/local/opt/rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/html-pipeline-2.2.1/lib/html/pipeline/@mention_filter.rb:106:in `mention_link_filter'

After

> filter = HTML::Pipeline::MentionFilter.new nil
=> #<HTML::Pipeline::MentionFilter:0x007fc76b019688 @doc=nil, @html=nil, @context={}, @result={}>
> filter.mention_link_filter "@hubot"
=> "<a href='/hubot' class='user-mention'>@hubot</a>"

/cc @brittballard

[jch]

@benbalter nice catch, thanks!