Searchstims

Added
- colors available for 'rectangle' stimulus that move either from red or green towards yellow (because of how
RGB color space works); this makes it possible to vary the discriminability of the target and distractor along a
single feature dimension (color). Using this one can fit a psychometric curve to the classifications of a neural
network and estimate its accuracy level for a known discriminability of targets from distractors.

Changed
- image filenames in .json file are saved as relative to `output_dir` that was specified in `.ini` file
when the images were generated by `searchstims`. This way it is possible to move the entire output directory
without breaking anything, but it does require that any scripts using the `.json` file know the path to that
output directory, and join the filenames to it (e.g. using `os.path.join`)

Added
- ability to have items located randomly (not on a grid), and specify a minimum center
distance between all items when placing randomly

Added
- ability to set "border" within window, so that items are not placed outside
the border. This provides another way to test for edge effects, i.e. by seeing
whether impaired accuracy is rescued when items are kept within the borders

Fixed
- data files (such as .png images of numbers) are now installed correctly

Added
- More detail about `config.ini` to README.md, explains what the options actually are for
- `.json` file saved by searchstims has more info about stimuli, such as location
of targets and distractors (in case this is needed for analysis)
- unit tests

Changed
- separate `main.py` into functions, to separate concerns;
* the `main` function deals with command-line args
* `parse_config` loads and parses `config.ini` files
* `make` function accepts parsed config
* This way you can call the `make` function with a `ConfigTuple` instance
regardless of its origins (e.g. if you need to write tests for `make` )
- command-line interface now requires name of config file as first arg, instead of
specifying it with optional `-c` flag (vestige of when I didn't understand how
`argparse` is meant to work)

Fixed
- indent error in `main.py` that caused crash when `stimulus = rectangle`