Add custom extensions support

[austinkregel]

Changes

• Adds the ability to add custom file extensions to the search for files with the flags --extentions=php,html or -e php,html

Closes #5

[abdumu]

@austinkregel

Could you fix the conflicts and also, filter the given extensions. like using array_map('trim', ..) then array_filter(...). Also it will be great to check if the given extension starts with a dot and remove it.

These are suggestions. You could just fix the conflicts and I would make the rest.

[austinkregel]

Like I said I'm at work at the moment, so I won't be able to until tonight, but I sure can :)

[austinkregel]

@abdumu I believe that's everything :) Is there anything more I should add or change?

[abdumu]

@austinkregel great, thanks 👍🏼