Html2image

- Changed the project structure to make it easier and cleaner to add new browsers in the future.
- Added a way to fully chose browser flags and change default ones.
- Updated the readme accordingly. Also added an FAQ section.

Fixing an issue preventing resolution change when taking screnshots using the `html_str` parameter.

fix 12: Process no longer hangs when using an incorrect size parameter, it raises a ValueError instead.

Fix 5: special characters weren't written properly to temp file when using the `html_str` parameter of the `screenshot` method.

Added the `custom_flags` parameter to the constructor.

This release induces backward incompatible changes.
- The `screenshot` method has been completely reworked to hide all the "loading" behaviours and can handle itself most of the use cases.
- All the old methods still exist but some have been renamed.
- Changed class name from HtmlToImage to Html2Image
- Refactored the CLI to use the new screenshot method
- New, and clearer, README
- More tests