Fix#180
index.initialBlacklist: ["."] can inhibit initial indexing (useful for larger code bases).
Opened files are still indexed, though.
This heuristic allows related files (a/foo.c a/b/foo.cc) to be indexed when a header (foo.h) is opened.
* Add %h for C header files (the suffix .h is considered a C header, not a C++ header)
* Add %hpp for C++ header files
* If .ccls exists, it provides full command line for files not specified by compile_commands.json (before, compile_commands.json was ignored)
* If the first line of .ccls is %compile_commands.json, it appends flags to compile_commands.json "arguments", instead of overriding.
Files not specified by compile_commands.json will not be added to folder.entries, but their command line can be inferred from other files.
Also fix `#include <` completion of -I flags for clang < 8
