Add --indent option

I'd like to post these to forums that use markdown, where monospace fonts are used if you have four spaces of indentation. It'd be nice if the tool had this option, but also you could indent any amount to right-justify the moon instead of having it be on the left.

Start by locating the command-line entry point and the logic that renders the moon phase. Implement the requested --indent behavior so output can receive a user-specified amount of leading whitespace, including four spaces for Markdown monospace formatting, and verify the resulting alignment manually.

Written by the indexing model from the issue text.