Added initial defaults array to initiate #10
Add filter for setting defaults:

Usage:
```
function set_pico_defaults( $defaults ){

	$defaults['custom_meta_options']['box_1'] = false;
	$defaults['custom_meta_options']['box_2'] = false;
	$defaults['custom_meta_options']['video'] = false;

	return $defaults;
}

add_filter( 'pico_slider_set_defaults', 'set_pico_defaults' );
```
Add filter for altering Slider CPT details
We haven't needed to alter the labels yet so let's leave that as is
Remove duplicate menu_icon
Refactor the CPT filter
Prefix the variables with cpt as we'll be adding more variables for the image
Refactor image size filter
Refactor the meta box filter
Make sure our description is translatable
Minor code formatting
Pass the meta filters into the saving function
Update our inline docs
Make sure we flush rewrite rules on activation and deactivation
Bump the version number
Add a blank index to stop directory browsing
Exit if the file is directly accessed
Add a licence and a Github URL for the Github updater plugin