Support backslash line continuation in crontabs

[ChrisJr404]

Fixes #104.

This adds support for splitting a long crontab entry across several lines by ending a line with a backslash, like you can in a shell. The parser joins the continued lines back together before it works out the schedule and command, so this:

*/10 * * * * echo 'a' && \
             echo 'b' && \
             echo 'c'

now parses as a single job instead of being rejected.

A trailing backslash that is itself escaped (an even number of backslashes) does not continue the line, and the join happens after the comment check so a comment ending in a backslash can't swallow the job on the next line. I added test cases for the basic case, several continuations in a row, an env definition, the escaped-backslash case, and the comment guard, plus a short note in the README.

Happy to adjust the join behavior (for example trimming the leading whitespace on continued lines) if you would prefer something different.