Download by URL and file input

What's Changed

• Download Channel/Thread by URL by @rusq in #26

Features

• Downloading Channels/Threads by URL
• Input file support via -i file.txt
• Verbose output with -v command line flag
• User caching, we don't request users from API every time we restart, we cache them instead, this can be controlled with -user-cache-age and -user-cache-file parameters.

Fixes

• Update doc
• Account for unsuccessful API response in Channel D/L (but nil error).
• Unexport URLInfo, along with parsing.

Misc

• Test coverage 15.4% -> 73.5%
• Simplify Channels type
• Add more tracing

Known issues

Sometimes we get rate limited more than three times in a row when downloading channels, this will be fixed in v1.3.1.
Workaround: set -limiter-boost=0 command line flag.

Full Changelog: v1.2.1...v1.3.0