buble theme: missing generic fallback font

The buble theme does not specify a generic font. Code blocks are displayed with a variable width font if Inconsolata is not available.

https://github.com/docsifyjs/docsify/blob/86f134d13254286efa6f4dfe7a8969c0b50b8bb2/src/themes/buble.styl#L162

Add `monospace` to all font-family usages?

`font-family Inconsolata, monospace`