Add --filter flag

The normal approach to filtering would be to just use Unix pipes.
That strategy doesn't work well here, because the histogram max width is
determined by the line with the most commits. So if one file has been
changed 1000 times and all the rest are orders of magnitude smaller,
the bar chart will have a lot of unused space.

git-heatmap

@@ -31,6 +31,11 @@ while [[ $# -gt 0 ]]; do
       shift
       shift
       ;;
+    -f|--filter)
+      FILTER="$2"
+      shift
+      shift
+      ;;
     -h)
       cat <<EOF
 Heatmap of oft-edited files.
@@ -44,6 +49,8 @@ Options:
   -b <branch>, --base <branch>  Compare relative to <branch>. If on <branch>,
                                 show heatmap for entire repo. [default: master]
   -c <char>, --char <char>      Use <char> to draw the bars. [default: █]
+  -f <cmd>, --filter <cmd>      Filter output through <cmd> before creating the
+                                the histogram.
   -h                            Show this message.
 EOF
       exit
@@ -59,6 +66,7 @@ LIMIT=${LIMIT:-30}
 REVIEW_BASE=${REVIEW_BASE:-master}
 CHAR=${CHAR:-█}
 WIDTH=${WIDTH:-60}
+FILTER=${FILTER:-cat -}
 
 files() {
   # https://stackoverflow.com/questions/7577052/
@@ -76,6 +84,7 @@ color_name() {
 
 filter() {
   grep '.' | \
+    eval "$FILTER" | \
     sort | \
     uniq -c | \
     sort -nr | \