Implements custom image support in the Match object, and adds support for it and also icons in the Stdin plugin (without breaking anything)

Feel free to point out anything I should change/forgot, a few key things I couldn't decide on myself:

• Maybe merge this feature into the icon field of Match somehow?
• Should hide_icons also hide images? Or should images always be shown? Or should they have their own switch? I went with the first option because custom images are supposed to override icons, sort of.
• This is not really up to decision, because GTK CSS does not support changing the size of GtkImages, but I had to go with having a max width and height in the config file instead.
• The default values for the 2 options mentioned above are undecided, I put 150 for max width, and 100 for max height.

For the Stdin part of it:

• The title and the image path is currently separated by a tab character, is there a better way of separating data?
• How should I go about also adding support for normal icons in Stdin? (see Stdin readme for how it is currently done)

Breaking changes:

• Plugins need to add the new field (avoidable if the image field is merged into the icon field)
• (Not really) 2 new config fields
• Can't pipe tabs into the Stdin plugin now (maybe avoidable?)

Closes #126