Add a proper logger to fetch

[robmorgan]

This PR introduces a proper logger to fetch to ensure it's output is in line with other Gruntwork tools. It also makes integration testing much easier and solves the following issues:

• 30

• 89

Note: This PR must be merged after #88 as it improves upon it.

Design Notes

• I noticed a lot of methods required multiple string params instead of a single FetchOptions struct, so I’m directly passing in a logger to follow the same pattern in most methods.
• I added the logrus library but only saw the macOS binary increase by 100kb from 8.9M to ~9.0M.

Progress

• [x] Replaced all of the fmt.Printf calls.
• [x] Introduced the logrus library.

[robmorgan]

Nice, thanks Rob!

We should probably add a --log-level param so you can control the amount of logging.

Thanks! Yeah, I intentionally avoided that and instead defaulted to 'INFO'. I wasn't sure if it made sense because I would either suppress a lot of the existing messages by default or we'd need to sprinkle in a few debug messages around the place to actually make it useful. I thought it would be easier to leave it out to under complicate this PR, but I agree it's something we'd want in the future. I could add it but default it to 'INFO' then people can override it to 'ERROR' or 'DEBUG'. What do you think?

[robmorgan]

@brikis98 okay I've added a --log-level option and ported the logging code over to gruntwork-cli. To actually make the --log-level option worthwhile I've sprinkled in a few DEBUG log entries, so execute fetch using --log-level debug to see them.