Feature: Add support for screencapture

[pbnj]

Adding support for most common screen capture use-cases might be nice.

For example:

• To screen-capture a Window: screencapture -iW ~/Desktop/screen.jpg (where i flag is interactive and W is Window Selection Mode)
• To screen-capture a user-defined rectangle: screencapture -i ~/Desktop/screen.jpg
• To screen-capture the whole screen: screencapture -S ~/Desktop/screen.jpg

I envision the CLI interaction might look something like:

usage: m screencap [ file-name | -w | -f | -r | help ]

Examples:
    m screencap                          # full screencapture of current screen. File saved as ~/Desktop/screen.jpg
    m screencap window.png -w            # prompt user to select a window to screencapture
    m screencap ~/Documents/full.jpg -f  # full screencapture of current screen
    m screencap rectangle.pdf -r         # prompt user to select custom dimensions.

Gotchas:

• File name is required. If user types any command without specifying a file name, use a default name and location (e.g. ~/Desktop/screen.png)

Reference: http://guides.macrumors.com/Taking_Screenshots_in_Mac_OS_X

[rgcr]

That's a good idea 👍

[thegranddesign]

Options for screen capture are implemented in #102