adjust excerpt function to make suffix a keyword argument

[jeremybmerrill]

This PR would extend the existing excerpt function to take an optional keyword argument suffix= that would be appended to the end of the excerpt, instead of the current ... ellipsis. If the argument is unspecified, the ... suffix will be retained, i.e. this change is backwards compatible.

For instance, if suffix is set to an empty string, the excerpt will be just the first N characters of the string. ``{{excerpt string 10 suffix=""}}returns'That can o'` and `{{excerpt string 10 suffix="asdf"}}` returns `'That can oasdf'`.

I have added tests; all pre-existing tests pass unmodified.

For context: just-handlebars-helpers is packaged with Apache Superset's handlebars charts. I need to do some string processing there and want to use excerpt without the ellipsis.

Notes:

1. The keyword argument code is copied from the sprintf implementation.
2. If you'd prefer an implementation where suffix is an optional positional argument, you can revert the last commit in this PR.