Added quiet flag to hide progress animation during install

[artesmichael]

Adds a new flag to the install command which hides the progress animation from stdout.

Issue: #14

[cmcgee1024]

Thanks for your contribution.

There are other progress animations, such as self-update. I wonder if this should be abstracted out to one of the GlobalOptions in Swiftly.swift. But then, quiet is the opposite of verbose, which is already there so there will need to be some consideration about the interaction of the two. Perhaps they can be unified into a single flag and enum.

In terms of the progress animation. It already appears to be able to detect whether it is running in a terminal context or not, and falls back to a line-oriented output. Are you wanting it to display no progress at all when this option is set? Maybe the detection isn't very effective and leaks escape codes in certain logs? Any other things that should be quieted in the various swiftly commands when the flag is set?