Adds support for options to CLI and improves usability

[DaRosenberg]

This PR adds the following improvements to the CLI experience:

• CLI now exposes options for most of the LiquidOptions properties
• Template can now be passed using --template option (inline or as path)
• Context can now be passed either using --context option (inline or as path) or piped through stdin
• Output can be written to a file using --output option (default is to write to stdout like before)
• Legacy mode is retained in which template is piped through stdin to retain compat; should be removed for 11.0
• CLI now validates and prints argument errors
• CLI now supports a --help argument to print usage information
• Docs and setup tutorial are updated accordingly

The --template option is necessary to distinguish legacy mode, but can be removed in favor of a nameless argument for version 11.0, so the syntax would become for example:

npx liquidjs 'Hello, {{ name }}!' --context '{"name": "Snake"}' --js-truthy

Fixes #583
Fixes #584
Fixes #585

[DaRosenberg]

@harttle Any idea when you might get a chance to either merge this one, or let me know if further changes are required?

Reason I ask is, we need this functionality, and if it will take several weeks to get it into a release, then I think we may need to think about publishing our own wrapper package for this instead.

[harttle]

Sorry I thought I had submited review but actually not. It won't take weeks, I can follow this PR every day or two.

[github-actions]

🎉 This PR is included in version 10.5.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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