Fix error thrown on empty keys

[mdhgriffiths]

My project was failing to load config.md as an exception was thrown when parsing empty headings.

TypeError: Cannot read property 'undefined' of undefined
at put (***/node_modules/gulp-styledown/node_modules/styledown/lib/mdconf.js:91:28)
at ***/node_modules/gulp-styledown/node_modules/styledown/lib/mdconf.js:45:9
at Array.forEach (native)
at module.exports (***/node_modules/gulp-styledown/node_modules/styledown/lib/mdconf.js:31:8)
at exports.processConfig (***/node_modules/gulp-styledown/node_modules/styledown/lib/filters.js:95:16)
at Object.Styledown.process (***/node_modules/gulp-styledown/node_modules/styledown/index.js:212:5)
at Object.Styledown (***/node_modules/gulp-styledown/node_modules/styledown/index.js:106:8)
at Function.Styledown.parse (***/node_modules/gulp-styledown/node_modules/styledown/index.js:59:10)
at DestroyableTransform.flush [as _flush] (***/node_modules/gulp-styledown/index.js:55:24)
at DestroyableTransform.<anonymous> (***/node_modules/gulp-styledown/node_modules/through2/node_modules/readable-stream/lib/_stream_transform.js:135:12)