Feature Request: Allow for color customization

**Is your feature request related to a problem? Please describe.**
It is extremely difficult to read Taskbook's output with my terminal colors

**Describe the solution you'd like**
Because terminal styling is highly irregular, allowing users to configure the color hexes in the ~/.taskbook.json would be the ideal solution.

**Additional context**

This is on iTerm 2 Build 3.2.20180723-nightly in ZSH, with theme "af-magic"

Start by locating the terminal styling code and the configuration handling for ~/.taskbook.json. Define the configurable color values there, then verify that Taskbook output can use user-selected colors and remains readable with the reported terminal setup.

Written by the indexing model from the issue text.