Allow logging to a file

[skx]

In #122 we updated our codebase to ensure that we used a common logging handle to send our output messages to the user:

• This included warnings and errors by default.
• However "developer", or other internal messages, were also available.

This pull-request builds upon that work to write the log messages to a file, as well as showing them to the user. By default a file rss2email.log is generated when the application starts and log entries are appended to it.

However:

• LOG_FILE_PATH can be set to a different path.
• LOG_FILE_DISABLE can be used to disable this duplication.

Why do this? Partly for reference, but also partly to allow a local setup to view errors.

We could now resolve #119 by adding a cronjob:

    #!/bin/sh
    # show logs, and delete them, run once per day
    cat .../rss2email.log
    rm .../rss2email.log

(Of course this might not work 100% as the current approach assumes the file is open forever in the case of daemon sub-command.)

This isn't a complete solution, but without getting into the whole template-customization and more complex considerations it's not a terrible thing to do.