Fix hyphens in paragraphs treated as signatures

A double dash within a paragraph can be used to convey an en dash or an
em dash, so it shouldn't be treated as a signature.
The important change here is that hyphens, and underscores must end the
line, optionally preceded, or followed by spaces.

(cherry picked from commit e49c84d41ab8cd3346f5a0451137e862c3f4ac6c)

Conflicts:
	lib/email_reply_parser.rb
	test/email_reply_parser_test.rb