Fix PS2 and backslash continuation

[mathiasertl]

Fix backslash continuation and set PS2 to include a space (so > instead of >), which is the default for most (all) shells. Currently, lines that end with a backslash are always rendered verbatim, effectively breaking any actual examples that want to show the PS2 prompt correctly. Using a common bash session, this is how you would split a command into multiple lines:

$ ls --many options \
> /some/location

... where both $ and > would be the prompt (configured by the PS1 and PS2 environment variables). So naturally, we want these tokens to be shown as prompt (and also not be copy-able).

Current test cases also do not reflect this behavior. For example, the test_newline_in_echo has a fragment of '$ echo \\\nhi\nhi\n', which would be impossible to actually create (unless PS2 where configured to an empty string), as a user typing echo \<enter> would imediately get a > in the next line.

As an example, here is how this renders without the patch. Note how this is actually inverted: The > from the certbot command is not detected as a prompt (even though it actually it is), while for the echo command output is detected as a prompt, even though it actually it isn't:

Now with the patch... it's actually correct: