Can't process subplot documents that are in a different directory
I can't run `subplot docgen examples/seq/seq.subplot` when I'm at the root of the Subplot source tree:
~~~
07:23:01 > cargo run -q -- docgen examples/seq/seq.subplot -o seq.html
INFO: Starting Subplot
ERROR: Failed to read input file seq.md
ERROR: caused by: No such file or directory (os error 2)
~~~
issue