Searchstims

Changed
- refactor `searchstims.make` so it is more general
+ now accepts a `stim_dict` that maps visual search stimulus names to instances of `StimMaker` classes
+ in this way the function can be used with a script that passes user-defined `StimMaker` sub-classes
- instead of being limited to the two types of stimuli that can be specified in the `config.ini` files
+ the alternative would've been to keep adding new possible sections the `config.ini` but this would've
been gross and painful

Fixed
- fix how metadata is saved now that all sections in a single config.ini file are made
+ `searchstims.make` function adds a key for each section to the `dict` it uses to
track stimuli, so the `.json` file created from that `dict` has metadata for each section

Added
- [GENERAL] section of config has `enforce_unique` option
+ makes sure all images are unique when a grid is used
+ option is True by default
- added `imageio` as a dependency
+ needed to open .png files for tests
+ will be needed for other tests that look at actual images generated whenever they get written anyway

Added
- possible to specify options that are common to all stimuli in [GENERAL] section
of config.ini file
+ e.g., image_size, item_bbox_size, jitter, grid_size
+ if one of these options is also specified in a section for a specific stimulus,
that value overrides the one specified in the general section

Changed
- user specifies an item bounding box for items in stimulus, instead of 'rect'
+ the actual item is drawn within this "bounding box"
+ would make it possible to draw items with different sizes but same shape
- user specifies sizes in order of (height, width)
+ since people feeding images to neural networks are used to this order
+ even though PyGame expects (width, height)

Added
- possible to make multiple types of stimuli with a single config file
+ any section that is defined gets made

Changed
- clean up configuration handling
+ add `searchstims.config` sub-package, move config-related code in there
+ uses `attrs`-based classes instead of `NamedTuple`s

Fixed
- continuous integration no longer fails, tests pass
+ set environment variable for library that PyGame depends on so that library runs with a dummy driver
and that way the "no video card" error doesn't get thrown

Added
- Acknowledgments with number for DARPA funding in README.md.
- Switch from pre-release to release to get a DOI for version 0.2.0