feat: set medium colour to yellow (#306)

As discussed in gotwarlost/istanbul#542, this
PR brings back the yellow colour for medium.

There are many good points raised in that PR, and this PR, aims to
replicate address those concerns for this repo.

packages/istanbul-reports/lib/html/assets/base.css

@@ -172,16 +172,17 @@ table.coverage td span.cline-any {
 /* dark green */
 .status-line.high, .high .cover-fill { background:rgb(77,146,33) }
 .high .chart { border:1px solid rgb(77,146,33) }
-
-.medium .chart { border:1px solid #666; }
-.medium .cover-fill { background: #666; }
+/* dark yellow (gold) */
+.status-line.medium, .medium .cover-fill { background: #f9cd0b; }
+.medium .chart { border:1px solid #f9cd0b; }
+/* light yellow */
+.medium { background: #fff4c2; }
 
 .cstat-skip { background: #ddd; color: #111; }
 .fstat-skip { background: #ddd; color: #111 !important; }
 .cbranch-skip { background: #ddd !important; color: #111; }
 
 span.cline-neutral { background: #eaeaea; }
-.medium { background: #eaeaea; }
 
 .coverage-summary td.empty {
     opacity: .5;