Should allow for custom goal types so the user isn't stuck with the default weekly/monthly/yearly/other set

Right now, the CLI replaces the full README every time a change is requested and inserts the change in the new content. This means I can't change the boilerplate content of the README, since it's just going to revert back to the initial state when I change/add/remove a goal.

Instead, the first time the CLI is run and the README would be adapted it should create a new README based on the template, with a small change: it has invisible markers for goals. Something like this:

Personal Goals
==============
Personal goals made open source for accessibility across computers I use, transparency, accountability, and versioning. Learn more about it [here](http://una.im/personal-goals-guide).

# Overarching Goals for 2017:
### This year's focus: Do good things this year

  
<!-- personal-goals-cli yearly -->
  
# Weekly Goals Aug 27th, 2017:
### This week's focus: Do good things this week

<!-- personal-goals-cli weekly -->

# Monthly Goals August 2017:
### This month's focus: Do good things this month

<!-- personal-goals-cli monthly -->

# Other Goals:

<!-- personal-goals-cli other -->

(notice the HTML comments; those won't show up on GitHub)

Now, rather than replacing the whole README when a goal is added/changed/removed, it just looks for the marker and adds/changes/removes the goal there. That way, I can change the format, edit the top part of the README or add more information add the bottom but the goal setting will still work!

What do you think?

Now that folks can edit the README I think it'd only be fair to add a note that they're using this CLI below the opening paragraph. Good marketing!