This reverts commit c13f72ee0a6600963a10e57155d2e814d56599e2.

References #5447.

We use a Makefile which in turn uses cmake. If we wanted to set the install
prefix for cmake, we had to do this so far:

  make CMAKE_FLAGS="-DCMAKE_INSTALL_PREFIX=/tmp/nvim"

That's long and hard to remember. Following the conventions of other Makefiles,
this now works as well and is equivalent:

  make PREFIX=/tmp/nvim

- Check if MIN_LOG_LEVEL value is a number 0-3, default to
  INFO (1) or ignore it in Release mode
- When TRAVIS_CI_BUILD is ON the default is DEBUG (0)
- Add local.mk.example

It hasn't been used in quite a while, so it's probably safe to move it
out of the repository root.

"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.

Introduce new build type Dev that replaces RelWithDebInfo for development
builds off master and has optimizations, debug info, and logging enabled.
Keep assertions enabled for RelWithDebInfo.

This doesn't prevent compilation of jemalloc, but the important thing is
that it prevents it being used in nvim.

refs 6cd20177df9d675c4c8de9ae27ce8c0ea2a94cca
refs 95707bf336794c7ec8991917f184c02ae873dd8c

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).