provider: Major refactor

- Providers for features are now registered as a unit. For example, instead of
  calling `register_provider("clipboard_get")` and
  `register_provider("clipboard_set")`, clients call
  `register_provider("clipboard")` and nvim will assume it implements all
  methods of the "clipboard" feature
- Bootstrapping code was removed. With the `api_spawn` function exposed to
  vimscript, it's no longer necessary and will be handled by plugins
  distributed with nvim.
- Now the `has` function will return true if there's a live channel that
  has registered as a provider for the feature.
- 'initpython'/'initclipboard' options were removed
- A new API function was exposed: `vim_discover_features` which returns an
  object with information about pluggable features such as 'python' or
  'clipboard'

src/nvim/option.c

@@ -974,12 +974,6 @@ static struct vimoption
   {"infercase",   "inf",  P_BOOL|P_VI_DEF,
    (char_u *)&p_inf, PV_INF,
    {(char_u *)FALSE, (char_u *)0L} SCRIPTID_INIT},
-  {"initclipboard","icpb",P_STRING|P_VI_DEF|P_SECURE,
-   (char_u *)&p_icpb, PV_NONE,
-   {(char_u *)"", (char_u *)0L} SCRIPTID_INIT},
-  {"initpython","ipy",P_STRING|P_VI_DEF|P_SECURE,
-   (char_u *)&p_ipy, PV_NONE,
-   {(char_u *)"", (char_u *)0L} SCRIPTID_INIT},
   {"insertmode",  "im",   P_BOOL|P_VI_DEF|P_VIM,
    (char_u *)&p_im, PV_NONE,
    {(char_u *)FALSE, (char_u *)0L} SCRIPTID_INIT},