New occurences method

[emilos]

Acceptance criteria

• occurences method in collection.js
• works for arrays (e.g. occurences(["foo", "foo", "bar"], "foo") returns 2)
• works for strings (e.g. occurences("foo foo bar", "foo") returns 2)
• tests
• docs
• examples

[inikonorov]

how this method must work with strings? can I consider that repeated substrings always separated by space?

[emilos]

@inikonorov that should be good enough for start, there's always room for improvement in the future if needed

[inikonorov]

done!
#85

[emilos]

@inikonorov thanks for all of the help! I've released it as v1.1.16