feat: Allow custom color palettes for XString objects

[ahl27]

Adds a collection of functions that allow users to specify new color palettes to display XString objects. This solves #115.

Slight refactor to move global variables into a package-specific environment to remove some <<- calls.

Also allows for users to specify colors for BString objects if they'd like, though the default is still no coloring.

Adds some additional unit tests and documentation pages.

I'm not sure I love the update_X_palette format for the function names...maybe updateDNApalette or update_DNA_COLORED_LETTERS would be better? Unsure.

Example usage is below.

DNA_palette <- list(
  A=list(fg="blue",bg="black"),
  T=list(fg="red",bg='black'),
  G=list(fg='green',bg='black'),
  C=list(fg='yellow',bg='black')
)
AA_palette <- list(
  A=list(fg="white", bg="purple"),
  B=list(fg=rgb(1,1,1), bg='orange')
)
B_palette <- list(
  A=list(bg='green'),
  B=list(bg="red"),
  C=list(bg='blue'),
  D=list(fg="orange"),
  E=list(fg="yellow")
)