Add option to increase specificity of class for styled components

[nvanselow]

This PR works with QuickBase/styled-components#3

It allows the specificity class to be set through a babel plugin configuration.

{
  "plugins": [
    ["babel-plugin-styled-components", {
      "namespaceClasses": 'moreSpecific'
    }]
  ]
}

or for a list of classes

{
  "plugins": [
    ["babel-plugin-styled-components", {
      "namespaceClasses": ["moreSpecific", "verySpecific"]
    }]
  ]
}

Results in classes to be pre-prended with the specificity class.

E.g.,

.moreSpecific .mx12n49 {
  background-color: yellow;
}

[harrismd]

Usually these are called "namespace classes" (not to be confused with CSS namespaces which serve a different purpose). Perhaps that terminology would be more clear.

[genedwards]

lgtm

[nvanselow]

Great name suggestion @harrismd! Fixed.

[harrismd]

Is this supposed to be: t.identifier('namespaceClasses')

[harrismd]

I assume this is accessing the value in the config...

[harrismd]

approved pending double-check of that one constant.

[nvanselow]

@harrismd - Fixed. Apologies that was a bit of a work in progress when you saw it. Had to make some other changes due to how babel-types work.