Approximate quadratic bezier curves with arcs and other minor improvements

[sivukhin]

I want to say, that I spent a great time exploring svgo and creating this little patch. Big thanks to all of the maintainers for this cool project

This PR introduces several changes:

1. Added two options: --apply-plugins-at-once and --report-profit-after-every-pass for the sake of exploring your current configuration of svgo and making it more performant by tuning parameters/adding new plugins/removing useless plugins that just waste computing time. If you enable both of these options you can see the following output in your console:

Iter. 1: 0.793 KiB - 0.9% = 0.786 KiB
    removeDoctype
Iter. 1: 0.793 KiB - 0.5% = 0.782 KiB
    cleanupNumericValues
Iter. 1: 0.793 KiB - 1.5% = 0.771 KiB
    removeUnknownsAndDefaults
Iter. 1: 0.793 KiB - 24% = 0.58 KiB
    convertPathData
Iter. 2: 0.793 KiB - 1.7% = 0.566 KiB
    convertPathData

2. Also, I fix a few bugs:
   a. reusePaths plugin can increase svg output size if there are no elements for reuse because this plugin always add <defs/> block
   b. reusePaths plugin always expect that first element of svg will be an <svg> tag, but this can be not true in case where we do not want to strip comments and meta-headers
3. Added sketch of the plugin part, that tries to replace a sequence of quadratic Bezier curves with circular arcs. This may look a little bit odd, because usually, vector graphics software represents arcs with cubic bezier curves, not quadratic, but I recently saw such example in gitter/sidecar repository and thought it will be nice to support such functionality in the svgo. Code for convertPathData looks a little bit scary for me and my patch with new functionality looks a little bit alien in this place of code. If you think that this bit of functionality is interesting than I can try to refactor loop with filters or create a separate plugin for the type of optimization