Refactor cli logic and add multiple features

[prime-run]

This PR refactors and redesigns the cli implementation.

Also implements the planned feature to export the generated HTML (#44),

Added Features :

• Adds the ability to export the rendered Markdown as a standalone HTML file.
• Changes the default command to standalone using $CACHE_DIR/go-grip/ path for html sources
• Introduces optional output dir -o , --output to exported static files to.
• Support for grouped md files rendering + a simple index page
• Update README.md accordingly
• Other small modifications

NOTE: as you see this PR has a single commit, the reason is I commited the built binary and after removing it's index from history (~root) realized I had the repo fully cloned (mistook my original clone for a --depth=1 as I alsmost always perform a shallow clone). And well you know what it means I guess!

Anyways in case you are intrested in actual commit history of this pr, here they are (after c446496 )

[chrishrb]

many thanks for this PR! Will look into it the next few days 👍

[ernstki]

Pretty sweet.

You can avoid breaking backwards compatibility with the existing way of running go-grip by implementing go-grip --export in place of go-grip render.

This is in fact the calling convention used by the original Python-based grip to write the rendered Markdown to a file.