add font-underline attribute (fixes #57)

quad/quad/scribblings/quad.scrbl

@@ -679,6 +679,10 @@ Whether the quad has bold styling applied. Activated only when value is @racket[
 Whether the quad has italic styling applied. Activated only when value is @racket["true"]. 
 }
 
+@defthing[#:kind "attribute" font-underline symbol?]{
+Whether the quad has an underline applied. Activated only when value is @racket["true"]. 
+}
+
 @defthing[#:kind "attribute" font-features symbol?]{
 Sets OpenType layout features. @racket[font-features] takes a @deftech{feature string}, which is an alternating list of OT feature tags and values, separated by white space. For instance, @racket{liga 0 smcp 1} would deactivate the ligature feature and activate the small-cap feature. If the feature string is prefixed with @racket["+"], rather than replacing the current feature settings, it amends the features that would otherwise apply.
 

quad/quadwriter/attrs.rkt

@@ -75,6 +75,7 @@ Naming guidelines
                font-features-adjust
                font-italic
                font-bold
+               font-underline
                font-tracking
                font-baseline-shift
                font-case

quad/quadwriter/string.rkt

@@ -76,6 +76,7 @@
      ;; whereas PDF drawing only puts tracking between the glyphs.
      (text doc str (+ x (/ tracking 2.0)) (- y (quad-ref q :font-baseline-shift 0))
            #:tracking tracking
+           #:underline (quad-ref q :font-underline)
            #:bg (quad-ref q :bg)
            #:features (quad-ref q :font-features default-font-features)
            #:link (quad-ref q :link))]))