Continue work on `write` command

Looks like @Schoonology is no longer working on his PR (#1148) so I wanted to
continue work on this stuff here.

Idea:


```bash
$ jekyll write post --title TITLE [--link LINK] [--layout LAYOUT] [--ext EXT]

$ jekyll write page --path PATH [--title TITLE] [--layout LAYOUT]

$ jekyll write post --template post --title TITLE

```