Add plugin config variables

1. Simplify usage of `AUTOSUGGESTION_HIGHLIGHT_COLOR` config variable (remove useless check for global variable is set).
2. Add config variable `AUTOSUGGESTION_HIGHLIGHT_CURSOR` to disable char under cursor highlighting (this char is not a part of entered command, but highlighted like it does, it is confusing a bit). I'd prefer to disable it by default, but this is your plugin and config variable is ok for me.
3. Add "Configuration" section to README file with description of config variables.
4. Remove spaces at the end of lines - simply beautify the code.