C/C++: a new field representing CPreProcessor context

For input.c:

```
#ifdef X
int v;
#else
float v;
#endif
```

What I expect:
```
v	foo.c	/^int v;$/;"	kind:variable	cppcontext:#ifdef X
v	foo.c	/^float v;$/;"	kind:variable	cppcontext:#else X
```

ctags has an implementation of an interval tree. So I guess implementing the cppcontext field is not so hard.
