Remove isDate

[leshakoss]

No description provided.

[YarnSphere]

Sorry to comment on this old pull request, but why was this functionality removed?
Calling instanceof Date fails when Date objects are passed across frames.
Other libraries such as lodash instead do something like:

typeof date === 'object' && Object.prototype.toString.call(date) === '[object Date]'

See lodash implementation (which seems to have a performance optimisation for Node.js): https://github.com/lodash/lodash/blob/master/isDate.js

I see no reason why this function shouldn't be provided by date-fns.
Thank you for your work!

[YarnSphere]

Just adding that a proper implementation of isDate would solve #541.
Calling instanceof is indeed not a good practice on a functional library such as date-fns since pure functions shouldn't access global state (and Date is a global variable).

Just leaving my two cents here.

[kossnocorp]

@nunocastromartins this is a very good point. @leshakoss WDYT?