enable packaging via setuptools

I couldn't get setuptools to use the root directory as
the source of the package, so I've moved everything to a
sub-folder, which is a workflow with better documentation.

A symlink is left behind to avoid breaking the setup for
people that install this to their pythonpath via git clone.

add option to disable automatic figure resizing

A user requested that there be an option to disable
automatic resizing to support certain workflows.

As accessing the actual backend instance is a bit awkward,
we expose this setting as an environment variable called
`MPLBACKEND_KITTY_SIZING`, named analogously to MPL's
`MPLBACKEND` variable used to select backends.

closes #3

fix: add support for matplotlib 3.3.0

On mpl 3.3.0, unfinished figures were being destroyed
before their subplots became available. This problem
resulted in only the background of a figure being shown.

We now also distinguish between interactive
and non-interactive matplotlib modes,
defaulting to interactive if `-i` is passed.

fixes #1

remove absolute positioning & fix displaying figures

It seems like kitty changed their handling of images somewhat.

The result is that the initial scrolling issues that lead to
prefering absolute positioning seem to be fixed, which means
we can simply use --align=left now.

Another side effect was that display of images was broken at
some point. This should be fixed as well with the above change.

Lastly, I've dropped transparency, as cell content now flows
properly around images, even in scrollback.

properly scale mpl figure to kitty's size

Note that the transparency and bg color settings used here
make some assumptions about your terminal's color scheme.