add assertStartsWith and assertEndsWith to complement assertContains

[go2null]

Add assertStartsWith and assertEndsWith to complement assertContains ( #98 )

If there is interest in this, I can submit a PR.

Intent is to use POSIX Paramter Expansion (https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02).

On that note, is there any reason why echo | grep was used for assertContains?

[go2null]

This covers the example use case in #101