add background colour enabler, and make some refactoring

1.Sometimes people just want underline only, without background colour, so I add background colour enabler.
2.As background colour and underline are selectable, I add makeLineStyleAttr method to deal with style attributes.  And when both of them unchecked, no need to update highlight line.
3.Even I want to remove the "enable" checkbox, because Atom setting page have own "Enable/Disable" button.  But I noticed you want to add some hotkey to enable/disable highlight line, maybe keep a main flag to control is good idea.
4.I think "enable" checkbox should be first, above the "backgroundRgbColor", so change the name to "AllEnable".