[feature] Add a flag to allow strip front matter

[at15]

This is for dyweb/weekly#186. When we are testing github workflow or manually creating more than one issues within a week, the content from those issues will be merged into one weekly markdown file. I plan to implement the merge logic in the go code in dyweb/weekly#187 and that requires getting the content without the front matter. I can strip it by matching --- but it's better if it can be supported by generator directly