Add support for Node 12

[Gasol]

Recently I have update Node to 12 through Homebrew on my Mac and notice the build of protagonist is broken, It seems like Node 12 removed the deprecated v8::Handle as following errors.

I get some hints from nodejs/nan#849, Read the docs of nan, and then try to fix it myself.

$ uname -a
Darwin Gasols-MacBook-Pro.local 18.5.0 Darwin Kernel Version 18.5.0: Mon Mar 11 20:40:32 PDT 2019; root:xnu-4903.251.3~3/RELEASE_X86_64 x86_64
$ node -v
v12.1.0

$ npm run build
gyp info it worked if it ends with ok
gyp info using node-gyp@4.0.0
gyp info using node@12.1.0 | darwin | x64
gyp info spawn make
gyp info spawn args [ 'BUILDTYPE=Release', '-C', 'build' ]

  ............... SKIP ..................

  CXX(target) Release/obj.target/protagonist/src/options_parser.o
In file included from ../src/options_parser.cc:2:
../src/protagonist.h:20:43: error: no member named 'Handle' in namespace 'v8'
    OptionsResult* ParseOptionsObject(v8::Handle<v8::Object>, bool);
                                      ~~~~^
../src/protagonist.h:20:60: error: expected '(' for function-style cast or type construction
    OptionsResult* ParseOptionsObject(v8::Handle<v8::Object>, bool);
                                                 ~~~~~~~~~~^
../src/protagonist.h:20:61: error: expected expression
    OptionsResult* ParseOptionsObject(v8::Handle<v8::Object>, bool);
                                                            ^
../src/protagonist.h:20:67: error: expected '(' for function-style cast or type construction
    OptionsResult* ParseOptionsObject(v8::Handle<v8::Object>, bool);
                                                              ~~~~^

  ............... SKIP ..................

10 errors generated.
make: *** [Release/obj.target/protagonist/src/options_parser.o] Error 1
gyp ERR! build error
gyp ERR! stack Error: `make` failed with exit code: 2
gyp ERR! stack     at ChildProcess.onExit (/Users/gasolwu/Code/protagonist/node_modules/node-gyp/lib/build.js:262:23)
gyp ERR! stack     at ChildProcess.emit (events.js:196:13)
gyp ERR! stack     at Process.ChildProcess._handle.onexit (internal/child_process.js:256:12)
gyp ERR! System Darwin 18.5.0
gyp ERR! command "/usr/local/Cellar/node/12.1.0/bin/node" "/Users/gasolwu/Code/protagonist/node_modules/.bin/node-gyp" "build"
gyp ERR! cwd /Users/gasolwu/Code/protagonist
gyp ERR! node -v v12.1.0
gyp ERR! node-gyp -v v4.0.0
gyp ERR! not ok

[kylef]

Thanks for taking the time to add Node 12 support @Gasol!