fix: Generate YAML safely by js-yaml

[nandenjin]

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

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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.