[Feature request] Create folder when using the output parameter

[michaelmass]

When using the parameter -output with a template like doc/{{.Dir}}/README.md, the cli returns an error if the folder doesn't exist.

failed to write output file doc\pkg\foo\README.md: open doc\pkg\foo\README.md: The system cannot find the path specified.

Maybe the cli should automatically create the folder or it could be a new flag -create-dirs.

I have already made a fork for it. If it is accepted, I will open a PR:
https://github.com/michaelmass/gomarkdoc/commit/b868c0fc4a6a07512eaf94f072f4325ba6efc709

[princjef]

Yeah that's a reasonable addition. It seems simpler to me to have it be the default behavior rather than a flag, unless you know of use cases where people wouldn't want that behavior.

I took a quick look at your change. Feel free to open a PR and we can get this merged/published.

[princjef]

Closed by #36