Add option to generate custom app config by name

[alin23]

Fixes #958

Problem

Creating custom app configs by hand is slow.

Solution

A single command to generate a config and populate it with relevant data based on the app name. The algorithm searches the following locations for filenames that contain the app name:

• ~/Library/Preferences
• ~/.config
• ~/ (only filenames starting with .)

Example:

> mackup generate hyper

will have the following output:

Creating /Users/alin/.mackup/hyper.cfg with the following data:

[application]
name = Hyper

[configuration_files]
.hyper.js

> mackup generate paste

will have the following output:

Creating /Users/alin/.mackup/paste.cfg with the following data:

[application]
name = Paste

[configuration_files]
library/preferences/com.wiheads.paste-helper.plist
library/preferences/com.wiheads.paste.plist