Add support for only running Download or Upload Test

[gaby]

When the host has a very slow connection, the user may not care at all about their upload or download speed.

If I want to do a quick test, I may only care about Download speed, similar to what fast.com does.

Example usage: --download, --upload, default to doing both

Related to #131

[code-inflation]

Good idea!

[code-inflation]

Implemented in v1.3.0

      --download-only
          Test download speed only
      --upload-only
          Test upload speed only

[gaby]

@code-inflation Thanks 💪