Create a function that sums two arguments together. If only one argument is provided, then return a function that expects one argument and returns the sum.
For example, addTogether(2, 3) should return 5, and addTogether(2) should return a function.
Calling this returned function with a single argument will then return the sum:
var sumTwoAnd = addTogether(2); sumTwoAnd(3) returns 5.
If either argument isn't a valid number, return undefined.
addTogether(2, 3) should return 5.
addTogether(23, 30) should return 53.
addTogether(5)(7) should return 12.
addTogether("https://www.youtube.com/watch?v=dQw4w9WgXcQ") should return undefined.
addTogether(2, "3") should return undefined.
addTogether(2)([3]) should return undefined.
addTogether("2", 3) should return undefined.
addTogether(5, undefined) should return undefined.