Add \overarc command.

This is simply another Style command with custom CSS to make curved border.
diff --git a/src/commands/math/commands.js b/src/commands/math/commands.js
index fdd3cf2..7d7cc31 100644
--- a/src/commands/math/commands.js
+++ b/src/commands/math/commands.js
@@ -81,6 +81,7 @@
 LatexCmds.overline = LatexCmds.bar = bind(Style, '\\overline', 'span', 'class="mq-non-leaf mq-overline"');
 LatexCmds.overrightarrow = bind(Style, '\\overrightarrow', 'span', 'class="mq-non-leaf mq-overarrow mq-arrow-right"');
 LatexCmds.overleftarrow = bind(Style, '\\overleftarrow', 'span', 'class="mq-non-leaf mq-overarrow mq-arrow-left"');
+LatexCmds.overarc = bind(Style, '\\overarc', 'span', 'class="mq-non-leaf mq-overarc"');
 
 // `\textcolor{color}{math}` will apply a color to the given math content, where
 // `color` is any valid CSS Color Value (see [SitePoint docs][] (recommended),
diff --git a/src/css/math.less b/src/css/math.less
index 389058c..ac6b777 100644
--- a/src/css/math.less
+++ b/src/css/math.less
@@ -343,6 +343,18 @@
     font-family: @symbola;
   }
 
+  .mq-overarc {
+    border-top: 1px solid black;
+    -webkit-border-top-right-radius: 50% .3em;
+    -moz-border-radius-topright: 50% .3em;
+    border-top-right-radius: 50% .3em;
+    -webkit-border-top-left-radius: 50% .3em;
+    -moz-border-radius-topleft: 50% .3em;
+    border-top-left-radius: 50% .3em;
+    margin-top: 1px;
+    padding-top: 0.15em;
+  }
+
   .mq-overarrow {
     border-top: 1px solid black;
     margin-top: 1px;