Add utility methods to check if the path exists

[sindresorhus]

This will be useful for #28.

I'm thinking findUp.exists() and findUp.sync.exists().

We already depend on https://github.com/sindresorhus/path-exists further down in the dependency tree, so we can just depend on that here without any added weight.

Should be tested and added to the readme and TS definition.

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[coreyfarrell]

Should this expose locate-path instead of path-exists? I ask since the new version supports checking for specific types of files and path-exists is no longer a dependency (not that it's a large dependency).

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