Fixes the error when adding external js files.

[nicolasbrugneaux]

Fixes this error:

❯ docker -o ../doc/ -i ./ --sidebar true --watch --js ../dist/externalLib.js

path.js:554
    throw new TypeError('ext must be a string');
          ^
TypeError: ext must be a string
    at posix.basename (path.js:554:11)
    at Array.map (native)
    at Docker.renderCodeHtml (/Users/nicolasb/.nvm/versions/io.js/v2.2.1/lib/node_modules/docker/src/docker.js:1283:22)
    at /Users/nicolasb/.nvm/versions/io.js/v2.2.1/lib/node_modules/docker/src/docker.js:408:18
    at next (/Users/nicolasb/.nvm/versions/io.js/v2.2.1/lib/node_modules/docker/src/docker.js:1086:37)
    at /Users/nicolasb/.nvm/versions/io.js/v2.2.1/lib/node_modules/docker/src/docker.js:1093:7
    at next (/Users/nicolasb/.nvm/versions/io.js/v2.2.1/lib/node_modules/docker/src/docker.js:1151:39)
    at Docker.highlighExtractedCode (/Users/nicolasb/.nvm/versions/io.js/v2.2.1/lib/node_modules/docker/src/docker.js:1165:3)
    at Docker.extractDocCode (/Users/nicolasb/.nvm/versions/io.js/v2.2.1/lib/node_modules/docker/src/docker.js:1129:8)
    at next (/Users/nicolasb/.nvm/versions/io.js/v2.2.1/lib/node_modules/docker/src/docker.js:1090:10)