Custom colorschemes

Added support for loading custom color schemes by path.

Added `ColorSchemeManager::loadCustomColorScheme(const QString& path)` which is a public method selecting loader based on the passed `path`'s extension.

`QTermWidget::setColorScheme(const QString& colorScheme)` now tries to guess if the passed color scheme is actually a path to a file, and, if so, tries calling `loadCustomColorScheme()`.