'iterm2' render style improvements

[AnonymouX47]

Closes #48

• Makes 'iterm2' style optimizations optional.
  • JPEG encoding.
  • Reading directly from file.
• Adds JPEG_QUALITY and READ_FROM_FILE attributes to ITerm2Image.
  • JPEG encoding quality can now be set.
• Adds --itjq/--iterm2-jpeg-quality and --itnrff/iterm2-no-read-from-file style-specific CL options.
• Adds 'iterm2' compress style arg and style-specific format spec field.
• Adds --itc/--iterm2-compress CL option.
• Fixes ITerm2Image.is_supported(), no longer returns None.
• Completes tests for the 'iterm2' render style.