Adds open command for quickly opening Svix's hosted docs

[codepope]

Always handy, svix docs will open the docs page in whatever browser is configured for the environment.

[svix-frank]

Always handy, svix docs will open the docs page in whatever browser is configured for the environment.

Hey @codepope thanks for the PR! I think it will be great to quickly access our hosted pages such as docs from the cli!

Just a few thoughts,

1. Since we likely will have more pages we'd like to be able to open in the future; I think we should break this out into a svix open command with docs as the first positional arg. That way we can add more pages easily without adding new functions (I can think of the dashboard as one we might want to add soon!). You can take a look at the completion function for a good example of how this might be implemented.

2. If we fail to open the URL we can print it as a terminal hyperlink so that users can still navigate to the page they requested (I have a bit of code commented at the end of pretty/print.go which you should be able to use for this =).

Let me know if you want any help with these suggestions or have any other thoughts!

[codepope]

Now has open command with docs or api as first parameters and a basic 'long' help to point the way
Also pretty prints the URL on fail to open browser. Small Todo; need to check in an uncommented pretty to use the hyperlink function there. Marked as such.

[svix-frank]

Thanks again @codepope for the PR!