let Message support markdown, text included; allow textcard btntxt to be omitted

[hophacker]

Wechat has supported markdown, which is much easier to write complex messages than other media types such as textcard, text.

Another reason of its importance is that, unlike textcard, text in which will be omitted once it's becoming a bit longer, one could write very very long text with markdown.

This commit added support for markdown. (Test included)

Besides that, this commit enables sending textcard without btntxt cause wechat corp provides default value 详情 if btntxt is omitted. (Test included)

All tests passed
Doc for markdown media type

[hophacker]

This is what markdown message looks like.