Fix default level padding in docs

Expose termcolor's ColorChoice and make colors optional

Rename time_format to filter_allow/filter_ignore where appropriate

Command line interfaces that write colored output usually give users
an option to turn it off, or make it automatically shut off when writing
to a file, pipe, or unsupported terminal. termcolor already has some
nice machinery to handle this sort of thing with ColorChoice, so expose
it when creating a TermLogger.

Programmers (such as myself) who use simplelog and want to give users
this choice only have ugly alternatives. For example, you could fall
back to a WriteLogger if the user doesn't want color, but then you:

- Lose out on the "mixed" logging mode you get from TerminalMode::Mixed

- Have to reimplement nice ColorChoice::Auto behavior yourself,
  on _top_ of termcolor.

White isn't a good default for those with light terminal screens
(brighter background, darker text). This is already trivial in
termcolor, so just allow level colors to be None.

Customizable level label colors

added `ConfigBuilder::set_level_color` to customize the colors of level labels for loggers that support color. added three examples showing the default colors, custom colors, and (if supported) RGB colors.

Remove unwrap() on TermLogger::new() in README.

Migrate to termcolor

Fix typo in config.rs

Fix typo on line 66

Derive useful traits for the terminal mode type.

rustdoc: hyper link to strftime.