Fixes option parsing on C++ libs that match EOL differently

[ttuggle]

The following fragment does not match between docopt.cpp and docopt:

Usage:
    myprog [options] <command> <arguments>...

Options:
  --reallylongoption <super_long_argument_description>
                This option fails to capture the argument on some
                C++ libs where $ does not match EOL.
                See https://stackoverflow.com/questions/39645660/stdregex-to-match-begin-end-of-string
                For some discussion and the inspiration to fix it by adding the \n.

[ttuggle]

FYI, this does include the code from #109

[ttuggle]

The pull request in #109 fixes problems parsing something like this, which does parse in Python.

usage:
    myprog [options] <file>


options:
    --fail      This arg unfortunately fails to process
                because there are two newlines preceeding
                the options section.

[jaredgrubb]

Is the issue that it won't match just a newline (without spaces)?

[jaredgrubb]

The |$ should match a newline right?