Parse input from a string definition of a function, into a tuple with {Module, function, arity}, so that it can be used for documentation lookup.

Examples:

iex> InputParser.parse("String.to_integer")
{String, :to_integer, nil}

iex> InputParser.parse("String.to_integer/2")
{String, :to_integer, 2}

iex> InputParser.parse("is_binary")
{Kernel, :is_binary, nil}

iex> InputParser.parse("IO.ANSI.Docs.print/2")
{IO.ANSI.Docs, :print, 2}

iex> InputParser.parse("InputParser")
{InputParser, nil, nil}

iex> InputParser.parse("IDontExist")
{IDontExist, nil, nil}