Add 'isSemver'

[CHE1RON]

hey there,
first of all: I really like this library, and although the long-term goal is feature parity with lodash, I'd like to propose a function checking whether a string represents a semantic version. If that sounds like something worth pursuing, I would like to contribute a PR 😉

Otherwise, just let me know briefly and close this issue, in any case

Cheers
CHE1RON