feat(template): better inline SVG post title support

BREAKING CHANGE: Now I dynamically include headline SVG based on post front-matter data `svg-headline`:

```
---
title: Hello World!
category: work
svg-headline: svg/post-title.svg
svg-headline-width: 400
svg-headline-height: 145
---
```

Note:

- The method to include SVG files has been changed since ([89d3dac341](http://github.com/sparanoid/sparanoid.com/commit/89d3dac3413e11e5139cf8c96e2adc490d27ad9a)), instead of embedding SVG into post files, now it include SVGs from external files using Liquid template tags `include`
- I use `for` loop here to include SVG file, it doesn't mean you can define multiple SVG header here, it's just a workaround since include files in a sub-directory with variable seems buggy: https://github.com/jekyll/jekyll/pull/1495
- In most cases, you have to define `svg-headline-width` and `svg-headline-height` in order to make your SVG title responsive to your viewport.