- Remove some unnecessary code: IsHeaderFile is no longer required, as the logic
  to find flags to headers is now built into YCM
- Add function to make paths in flags absolute: It seems YCM is not correctly
  resolving paths in flags to consider `build` as the compiler working
  directory.
- Update documentation.

"vimrc" refers to all files that are used to configure Neovim. The main
configuration file is init.vim nowadays.

All nvimrc references that are left refer to a local ".nvimrc" which is read
if 'exrc' is set. ".ngvimrc" references were completely wiped.

Closes #3552.

This setup is only meant for developers, not all users of YCM.

What works:

1. ShaDa file dumping: header, registers, jump list, history, search patterns,
   substitute strings, variables.
2. ShaDa file reading: registers, global marks, variables.

Most was not tested.

TODO:

1. Merging.
2. Reading history, local marks, jump and buffer lists.
3. Documentation update.
4. Converting some data from &encoding.
5. Safer variant of dumping viminfo (dump to temporary file then rename).
6. Removing old viminfo code (currently masked with `#if 0` in a ShaDa file for
   reference).

[ci skip]

Problem:  YCM was reporting a much disturbing warning about a missing
          newline at the end of some files. This was odd, as the
          newlines were there and the warning only was shown for some
          files, not for all of them.
Cause:    After discussing this issue with @Valloric (see
          https://github.com/Valloric/YouCompleteMe/issues/950), it
          turned out that not YCM, but libclang is responsible for it.
          This is, same compilation flags that produce no warnings with
          clang-the-binary on the command line, do produce them with
          libclang-the-library when used by YCM.
Solution: Add an extra flag (-Wno_newline_eof) to those extracted from
          configuration database before passing them to YCM.

Python-mode signaled a couple of issues here:
- Multiple imports on same line.
- Line too long.