Initialize asciidoctor.js only once

[amr]

Without this, only the first file works, subsequent streams fail:

[22:38:27] ArgumentError: illegal key name: block_terminates_paragraph
    at OpalClass.def.$backtrace.self [as $new] (docs/node_modules/gulp-asciidoctor/node_modules/asciidoctor.js/node_modules/opal-npm-wrapper/index.j
s:2783:17)
    at OpalModule.def.$raise (docs/node_modules/gulp-asciidoctor/node_modules/asciidoctor.js/node_modules/opal-npm-wrapper/index.js:2434:31)
    at OpalModule.a.cdecl.c [as $define] (docs/node_modules/gulp-asciidoctor/node_modules/asciidoctor.js/dist/npm/asciidoctor-core.min.js:10:5996)
    at a.cdecl.c (docs/node_modules/gulp-asciidoctor/node_modules/asciidoctor.js/dist/npm/asciidoctor-core.min.js:10:6233)
    at b (docs/node_modules/gulp-asciidoctor/node_modules/asciidoctor.js/dist/npm/asciidoctor-core.min.js:10:6601)
    at docs/node_modules/gulp-asciidoctor/node_modules/asciidoctor.js/dist/npm/asciidoctor-core.min.js:10:31491
    at Object.Asciidoctor (docs/node_modules/gulp-asciidoctor/node_modules/asciidoctor.js/dist/npm/asciidoctor-core.min.js:10:31548)
    at module.exports (docs/node_modules/gulp-asciidoctor/index.js:8:33)
    at Gulp.<anonymous> (docs/gulpfile.js:28:11)
    at module.exports (docs/node_modules/gulp/node_modules/orchestrator/lib/runTask.js:34:7)

[dongwq]

Thank you @amr , updated to npmjs.com