Option to auto-fix icon shading in light themes

Added functions that load, modify, save, and set color scheme files to counteract an issue in which color schemes with light backgrounds automatically darken PNGs, rendering gutter colors inaccurate.

Options are provided to scan & edit global user preferences, syntax-specific settings, and custom files (for users with other plugins affecting the current color scheme). This process only modifies the highest priority file that has set the current scheme, in the order custom, syntax, global, respectively (the `load_settings` command will automatically check defaults for each if no User version is set).

If enabled (defaults to off), the process will be applied to all views (with supported syntaxes) on plugin load, and to any such view when gaining focus. The process can also be triggered by a user command (which
will force the scheme to be regenerated even if it already exists, and ignore user disabling of global or syntax file scan).

Modified schemes are saved directly to the Gutter Color package directory with a special suffix.

For previous discussion see #57. Fixes #6. Fixes #37.