Prepare 0.3.0 release
Prepare 0.3.0 release

makyo/markdown-preview,ArnaudRinquin/markdown-preview,danielgtaylor/atom-api-blueprint-preview,tkssharma/markdown-preview,sctlee/markdown-preview,grimmer0125/markdown-preview-kramdown,Galadirith/markdown-preview,rugk/markdown-preview,atom/markdown-preview