Add changelog utility scripts (closes #172)

utils/generate-changelog.sh

+#!/bin/bash
+
+for tag in $(git tag -l --sort=-version:refname)
+do
+    pre=$(git describe --abbrev=0 $tag^ 2>/dev/null)
+    tag_date=$(git log -1 --format=%ad --date=short $tag)
+    echo "## [$tag] - $tag_date"
+    echo "### Added"
+    echo "### Changed"
+    echo "### Deprecated"
+    echo "### Removed"
+    echo "### Fixed"
+    echo "### Security"
+    git shortlog $pre..$tag
+done

utils/update-changelog.sh

+#!/bin/bash
+
+tag=$1
+
+pre=$(git describe --abbrev=0 2>/dev/null)
+tag_date=$(git log -1 --format=%ad --date=short)
+echo -e "## [$tag] - $tag_date\n"
+echo -e "### Added\n"
+echo -e "### Changed\n"
+echo -e "### Deprecated\n"
+echo -e "### Removed\n"
+echo -e "### Fixed\n"
+echo -e "### Security\n\n"
+git shortlog $pre..