Add scoped font sizes

[ArmorDarks]

Yet to be decided do we really need it.

We could allow to define specific for certain scopes font sizes, which will generated all needed sets of helpers classes.

Like that:

$ekzo-font-sizes: (

  text: (
    base: 14px, // used for vertical rhythm calculation; = .text--base, .font-size--base, .line-height--base
    '-':  12px, // = .text-,  .font-size-,  .line-height-
    '--': 10px, // = .text--, .font-size--, .line-height--
  ),

  headings: (
    h1: 36px, // = h1, .h1, .font-size--h1, .line-height--h1
    h2: 30px, // = h2, .h2, .font-size--h2, .line-height--h2
    h3: 24px, // = h3, .h3, .font-size--h3, .line-height--h3
    h4: 21px, // = h4, .h4, .font-size--h4, .line-height--h4
    h5: 18px, // = h5, .h5, .font-size--h5, .line-height--h5
    h6: 16px, // = h6, .h6, .font-size--h6, .line-height--h6
  ),

  extra-headings: (
    \+\+: 96px, // = .extra-h\+\+\+, .font-size\+\+\+, .line-height\+\+\+
    \+:   72px, // = .extra-h\+\+,   .font-size\+\+,   .line-height\+\+
    base:   44px, // = .extra-h\+,     .font-size\+,     .line-height\+
  ),

  buttons: (), // = .btn--*name*

  // --------> here magic happens
  blog: (
    text: 20px,
    headings: ()
  )
  // --------> here magic ends

) !default;