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.

- [x] Implement command
- [x] Implement `Template` class (or something similar)
- [ ] Write tests for `Template` class
- [ ] Write integration features for writing

Idea:

```bash
$ jekyll write post --title TITLE [--link LINK] [--layout LAYOUT] [--ext EXT]
# generates post at _posts/2013-08-07-title.EXT like this:
#
# ---
# layout: LAYOUT # (defaults to 'default', maybe?)
# date: "2013-08-07T23:14:13+02:00"
# title: TITLE
# [link: LINK]
# ---
#

$ jekyll write page --path PATH [--title TITLE] [--layout LAYOUT]
# generates page at PATH like this:
#
# ---
# layout: LAYOUT
# title: TITLE # if provided
# ---
#

$ jekyll write post --template post --title TITLE
# reads in contents of `_write/post.erb`, runs it through ERB and outputs it to the proper place
# it would also take all the options above and set each of them as variables,
#     thus `--layout LAYOUT` is accessible via `<%= layout # outputs "LAYOUT" %>`.

# Same thing with pages.
```