Fixes #5

Problem

• YAML syntax is broken by non-escaped template value (See Template values in yaml must be escaped #5)
• Unnecessary escape for some chars such as " or ': They are double-escaped by EJS compiler and VuePress

Proposal

• Use js-yaml to generate YAML safely
  • Inject safeDump() as toYAML() into EJS and use to generate YAML

I'm the very first developer for this repo so please tell me any problems with my code! I'll try to fix them immediately.