Support Indexed Arrays

This patch was authored and released by @siemensikkema.

Supports indexed array elements when decoding and encoding form data (fixes #67, replaces #69).

Indicating that a value belongs to an array can now be done by appending [0], [1], etc. to the name, as an alternative to the already supported []. This allows for representing nested data inside elements in an array.

For instance, let matrix: [[Int]] = [[42,21]] can now be represented using part names matrix[0][0] and matrix[0][1]. Without indexed array elements there is no way to distinguish the above from [[42],[21]].

Note: FormDataEncoder now also includes indexes for array types in its output.

Credits go to @chocoford for identifying the issue, the test cases and the initial PR!