Extend have_body_text matcher to be able to specify which part in multipart to check

[eitoball]

I would like to specify which part in multipart is checked by have_body_text matcher. In this PR, I added methods (?); .in_html_part and .in_text_part for have_body_text matcher. For example,

mail = Mail.new do
   html_part do
     body 'This is html'
   end
   text_part do
     body 'This is text'
   end
end

# The current behavior
expect(mail).to have_body_text(/This is html/)
expect(mail).not_to have_body_text(/This is text/)

# Using new methods created in this PR
expect(mail).to have_body_text(/This is html/).in_html_part
expect(mail).to have_body_text(/This is text/).in_text_part
expect(mail).not_to have_body_text(/This is html).in_text_part # Only look for text part

[lulalala]

bump @etagwerker

[winniehell]

@etagwerker can you please take a look at this?

[winniehell]

@etagwerker is there anything I can do to help get this merged?