feat: New option date for new post cli command

[tglman]

Hi,

this add a new option for provide a date when create a new post in the cli

Examples:

cobalt new "Last post" --date "2023-10-02 10:10:10"

cobalt new "Last post" --date "now"

bye

[epage]

Could you expand on why vs just editing the frontmatter directly?

[tglman]

Hi,

yes just for the option now that is actually quite useful, I initially thought to make the new just set the date to now all the times by default, without have the additional cli option, but this seems a bit more flexible, if you think is better I can do a PR with just setting the date to now without the option.