Add OS type detection and custom coloring for root user

[bokysan]

This commit includes these new features:

• A segment which displays OS type. This is especially useful
  if constantly logging into different computers with different
  systems and you need to know if you need to type apk, apt,
  dnf, or yum, for example. Enable it by setting the
  theme_display_currentos variable to yes.
• Padlock for SSH connection. You may enable the padlock by
  setting the theme_display_ssh_keylock to yes. Padlock is
  shown only when accessing the terminal via a SSH session.
  The padlock is displayed before the hostname.
• Split username and hostname segments. This enable each segment
  to be coloured with a different background. Set
  theme_display_split_user_hostname to yes to enable this
  feature.
• Color root username differently. When logged in as a root
  user, background (or the color of the username) will change
  to red. New colour has been added to the theme(s) for this
  feature.

[bobthecow]

so many changes at once! it's a lot easier to discuss them separately :)

[bokysan]

Kind of started fixing and ended up with a larger patch. :)

Even so, I don't think it's a lot of code changes. If you want, we can still split it, though, into separate patches. Any suggestions?