feat: Add support for swa-cli.config.json file

[alexweininger]

Feature:

This feature works as described by @anthonychu in #20 (comment).

• If only one configuration present in config file, running swa start will use this configuration.
• Otherwise you must use swa start <config name> to use a configuration. If <config name> is not found in the config file, no config will be used.
• Added the --config option, which allows the user to specify a specific path to a config file to use.
• Added the --print-config options, which prints the picked up configuration. Useful for user debugging.

I've updated the readme to document how to use a config file.

One thing to note: this feature can only either take options from the command line or the config file. It cannot merge or mix options. If a configuration from the config file is picked up and used no options passed in via command line will be respected.

This is a limitation of commander. More details at: tj/commander.js#1584

I've also created a JSON Schema for the swa-cli.config.json file.

Tests:

• Added unit tests
• Added e2e tests that function the same as the current e2e test, except use a config file rather than options passed in via terminal. Run these with npm run e2e:config

Closes #20

[manekinekko]

LGTM. cc @Reshmi-Sriram