Introduce Config and it's C FFI functions

[mominul]

This PR introduces Config to configure RitiContext and replaces the current weird way of configuring through environment variables.

Now to initialize a RitiContext, one has to pass a properly populated Config struct to the constructor function which is analogous to setting the environment variables before initializing a RitiContext.

This PR also introduces the following C APIs:

/*
 Config struct for configuring RitiContext.
 */
struct Config;

/*
 Creates a new instance of Config which is used to initialize
 and to control the configuration of RitiContext.
 This function creates an instance of Config in an initial
 state which can't be used before populating the Config using
 `riti_config_set_*` set of functions.
 */
Config *riti_config_new();

/*
 Free the allocated Config struct.
 */
void riti_config_free(Config *ptr);

void riti_config_set_layout_file(Config *ptr, const char *path);

void riti_config_set_database_dir(Config *ptr, const char *path);

void riti_config_set_phonetic_suggestion(Config *ptr, bool option);

void riti_config_set_phonetic_include_english(Config *ptr, bool option);

void riti_config_set_fixed_suggestion(Config *ptr, bool option);

void riti_config_set_fixed_auto_vowel(Config *ptr, bool option);

void riti_config_set_fixed_auto_chandra(Config *ptr, bool option);

void riti_config_set_fixed_traditional_kar(Config *ptr, bool option);

void riti_config_set_fixed_old_reph(Config *ptr, bool option);

void riti_config_set_fixed_numpad(Config *ptr, bool option);

Closes #12

[mominul]

cc @wengxt can you do a review, please?

[wengxt]

I don't use rust so I can't really comment about that, but the C API LGTM.

[mominul]

Thanks!