Add flags to prepend dates to Markdown filenames

[dge8]

A simple pair of flags to prepend exported note filenames with the created and/or modified dates (ISO format, YYYY-MM-DD, so they sort nicely in file managers).

Was useful for me, making the PR in case it's useful for others. However, a better long-term solution might be something like the proposal in Issue #58.

Cheers, Dan.

[wormi4ok]

Thanks for the contribution, @dge8!
I'm trying to keep the number of flags and configuration options to a minimum, and since the proposal in issue #58 covers many more use cases, I'd prefer to focus on that solution and not merge this one.

It will take time for me to work out the first proof of concept with the dynamic filenames. As well, it may require a new approach to the configuration. Multiple flags with complex template values seem to be hard to manage as CLI arguments.

So something like config files support might come up. Let me know if you have ideas on this topic!