Add jest --coverage -o to pre-commit script

The command '.node_modules/.bin/jest --coverage -o' would run jest to
test all files changed since last commit and print the report
(including code coverage) in the terminal.

The purpose of this commit is to enable collaborators to check if
the changed files are covered by unit-testing, without wasting time
running all the test suites.

This addition to the commit process might prove to be undesirable, and
is therfore subject to change.

.githooks/pre-commit

@@ -3,3 +3,7 @@
 set -e # using the options command to abort script at first error
 echo "running the 'pre-commit' script"
 ./node_modules/.bin/eslint .
+./node_modules/.bin/jest --coverage -o
+
+
+