Modified line completion callback to provide current position.

The line completion callback now provides the callback creator with the current
position in the line the cursor is at, so completions can done in all parts of
an unfinished line.

libline.c

@@ -465,7 +465,7 @@ static int complete_line(struct line_state *ls)
         int nwritten;
         char c = 0;
 
-        completion_callback(ls->buf, &lc);
+        completion_callback(ls->buf, ls->pos, &lc);
         if (lc.len == 0) {
                 line_beep();
         } else {

libline.h

@@ -57,7 +57,7 @@ extern "C" {
 #include <stddef.h>
 
 typedef struct line_completions line_completions;
-typedef void (line_completion_callback) (const char *, line_completions *);
+typedef void (line_completion_callback) (const char *, size_t pos, line_completions *);
 
 void line_set_vi_mode(int on);
 int  line_get_vi_mode(void);