Add animated arrow on tree when page is first loaded

[RhodesPeter]

• add arrow.svg to assets
• add addTextToElement function
• add hideElement function

Fixes #157 Related #8

[codecov-io]

Codecov Report

Merging #160 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #160   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines           9      9           
  Branches        4      4           
=====================================
  Hits            9      9

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 7fd8bd4...938f1aa. Read the comment docs.

[RhodesPeter]

I have changed this to addEventListener now @des-des. Thanks for spotting this.