* Move path handling out of conversion functions
This lets us decouple file reading/writing from the actual conversion.
* Move Config::from_args() to main.rs
* Remove path support from Config
Instead the input_path and output_path have to be passed to
convert_image_to_svg() manually.
* Add a simplified convert() function
This lets the user convert an image from memory, without encoding it to
PNG, writing it to a file, then reopening this file and decoding it
using the image crate.
It also allows the user to not write a SVG directly to a file.
