API for encoding and decode URL-encoded form data

[michaelrsweet]

Add an API for encoding and decoding application/x-www-form-urlencoded form data. Proposed API:

// Decode URL-encoded form data, returning the number of variables and a pointer to a
// `cups_option_t` array of the variables. Use `cupsFreeOptions` to return memory.
extern size_t cupsFormDecode(const char *data, cups_option_t **vars) _CUPS_PUBLIC;

// Encode options as URL-encoded form data, returning an allocated string. Use `free`
// to return memory.
extern char *cupsFormEncode(size_t num_vars, cups_option_t *vars) _CUPS_PUBLIC;

[michaelrsweet]

[master 2eced324f] Initial implementation of cupsForm APIs (Issue #49)

Still need to finish the unit tests.

[michaelrsweet]

[master 9a3855ed0] Implement unit tests.

W3C forms specification defining (poorly) the application/x-www-form-urlencoded MIME media type. Unlike URLs, anything not a letter or digit needs to be percent encoded (which turned up with the unit tests...)