Handle "From " lines

[jwodder]

• Give NormalizedDict a from_line attribute

• Give the scanner a from_line_regex parameter; if the first line of a stanza matches the regex, it is assumed to be a "From" line

• Create a "SpecialHeader" enum with FromLine and Body values for use as the first element of (header, value) pairs yielded by the scanner representing "From " lines and bodies

  • Use the enum values as keys in NormalizedDicts instead of having dedicated from_line and body attributes?

• Give the parser an option for requiring a "From " line

• Export premade regexes for matching Unix mail "From " lines, HTTP request lines, and HTTP response status lines