Handle `.` in project name

[tues]

Including a dot (.) in the project name causes the generated index.html file to break because the dot is not replaced with a dash (-): https://github.com/tues/scala-js.g8/commit/fe30379aacc3384850b635ace079a1114d8fc9d4#diff-39ce465d7a5231a284b09a7b9dcbaa13R9

For example, project name foo.bar should result in the above line being evaluated to:

<script type="text/javascript" src="../foo-bar-opt.js"></script>

Should be easy to fix (just find the correct Giter8 filter) but isn't critical.