Add color-override for Shell Highlighter

[johnedquinn]

May be useful to abstract the Token styling to a separate configuration a user can modify and provide through the ShellConfiguration. For accessibility (e.g. color-blindness), it may be helpful for a user to modify the Token styling to something other than the defaults. Feel free to create an issue to track since that configurability may be out of scope for this PR.

Originally posted by @alancai98 in #639 (comment)

[johnedquinn]

To give more context, with the new Shell Highlighter, we have the option to remove highlighting using --monochrome, but it would also be nice to provide an override to the colors. Right now, it is programmatic, but we should allow a way to pass in an Ion file to describe the colors of each token type.