Add native extension of JsThemis to "make fmt" target group.

This requires quite a bit of effort because clang-tidy insists on having
access to *all* source code involved in complation, including all those
NodeJS headers and its modules.

First of all, we have to add some hardcoded paths to NodeJS headers.
I have not found a proper way to get these paths so we just hardcode
them and hope that this is fine.

Then we need to have <nan.h> header file for which we have to install
the nan module. We do a little hack to achieve that, pretending to
'format' the installation path for Node modules.

This will make automated formatting a bit more nice.

Install npm and nodejs as we need them for checking JsThemis native
source code. At least for now.

Print out a warning when formatting files if NodeJS is not installed and
we are not able to reformat JsThemis files.

The current build system for libthemis-src is quite messed up and
requires peculiar symlinking to work. Add a symlink for the new file
included from the main Makefile.