Remove CLI and Plugin from Earmark --> EarmarkCLI

[RobertDober]

@pragdave I had been closing the issue #135 as it stroke me that it would add quite some functionality
to Earmark that is not used by our poweruser ex_doc when thinking about it I realized that the CLI and plugins part of Earmark too are completely unnecessary for them.

As a proof of concept, and because I need it ;), I just created https://github.com/RobertDober/EarmarkWrapper which allows to create customized wrappers around the html created by Earmark.

I would like to do one of the following three things, in order of precedence.

1. Deprecate the plugin and escript support in Earmark v1.3.2 and release EarmarkWrapper as EarmarkCLI with your blessing.
   As soon as EarmarkCLI is stable Earmark v1.4 can be leaned out.

2. Keep the escript and plugin support and just release EarmarkWrapper as a hex package if you do not mind me using this name.

3. Backport EarmarkWrapper into Earmark

Looking forward to your take on this.

[RobertDober]

I have created, but not released yet, another hex package which could render Earmark smaller.

Extractly

This is basically your readme task but using an .eex template and a CLI that allows to use any template and output.

However, again, I feel it is necessary to ask your permission to release this package as the core code is from you, which is duely reflected in the Copyright section of course.

[RobertDober]

Plugin has already been removed, however we will follow the following strategy to remove CLI, citing mail, before I forget

Dave:

I like both ideas, but I’d like to suggest a slightly different way forward. 
Rather than removing the earmark script from earmark itself, let’s reference these two libraries in the earmark README and then see if people adopt them. If they do, then we can think about stripping out Earmark functionality.

me:

Completely agree for the escript.
Good way to check the temperature.

However the README creation by extracting the docstrings does not concern our clients, it seems,
 I just had committed the error to add the `tasks` directory into packages[files:...]
 I will correct this.

Then replacing the task with the dev only dependency of extractly will be completely transparent and can be done later on.

[RobertDober]

The Plugin has been removed for quite some time now and the CLI will stay where it is as I have created the EarmarkParser

This means that the CLI will remain with the library for now