Conditional styles should be optional

[4513ECHO]

Currently, g:{colorscheme_name}_bold and g:{colorscheme_name}_italic are always defined. However, it is not good for me. I don't want to define global variables. This feature should be able to be configured by colorscheme authors.

I think, it is better than now:

colorschemes:
  - name: 'name'
    background: 'background'
    palette: 'palette'
    conditional_styles: true

By the way, conditional styles are not working currently. You have to use :execute such as:

execute 'hi SomeGroup gui=' .. Bold .. ',underlined cterm=' .. Bold .. ',underlined' 

instead of:

hi SomeGroup gui=Bold,underlined cterm=Bold,underlined