provider: Add support for python commands/functions

This uses the provider/scripting infrastructure to reintroduce python support
through the msgpack-rpc API.

A new 'initpython' option was added, and it must be set to a command that will
bootstrap the python provider the first time it's needed.

src/nvim/option_defs.h

@@ -630,6 +630,7 @@ EXTERN int p_write;             /* 'write' */
 EXTERN int p_wa;                /* 'writeany' */
 EXTERN int p_wb;                /* 'writebackup' */
 EXTERN long p_wd;               /* 'writedelay' */
+EXTERN char *p_ipy;             // 'initpython'
 
 /*
  * "indir" values for buffer-local opions.