Added check for max highlights groups, custom colors and shuffle colors

buildColors() function adapted from [jaxbot/semantic-highlight.vim](https://github.com/jaxbot/semantic-highlight.vim)

Assign custom colors in .vimrc through global variable
```
let g:interestingWordsGUIColors = ['#8CCBEA', '#A4E57E', '#FFDB72', '#FF7272', '#FFB3FF', '#9999FF']
```
Shuffle colors
```
let g:interestingWordsRandomiseColors = 1
```