Markdown deflist plugins

[ziqiq]

This patch integrates markdown deflist plugins(ebcff63 2020-09-10) and adjusts corresponding style.

Examples:

Definition - Undirected Graph
: An undirected graph is an ordered pair $G=(V,E)$ comprising
  - $V$, a set of vertices
  - $E \subseteq \{\{x,y\}| x \in V, y \in V, x \neq y\}$, a set of undirected edges

Definition - Walk, trail, and path
: A walk in a undirected graph $G=(V,E)$ is a sequence of vertices $v_1 v_2 ...$ where $\{v_i, v_{i+1}\} \in E$
: A trail is a walk where all edges are distinct.
: A path is a walk where all vertices are distinct.