Less mixin with optional parameters

To supply a parameter that far down the string of arguments, you have to also supply the expected variable it is to define. So this:

.fontStyle('NimbusSansNovCon-Reg', 12px, @letter-spacing: 0.1em);

Produces this (note how color, font-weight, and font-style used the defaults):

font-family: 'NimbusSansNovCon-Reg';
font-size: 12px;
color: #ffffff;
font-weight: normal;
font-style: normal;
letter-spacing: 0.1em;