Take inspiration from hamcrest's predicates

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See https://github.com/ujh/hamcrest-rust

Predicates in hamcrest not in here include

• regex match

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In assert-rs/assert_cli#85 someone was adding regex predicates to assert_cli. They had a desire for repetition matching. Something we should consider.

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#18 resolves the hamcrest part. Created a different issue for repetition.