Make import name configurable

[mxstbr]

Right now this processor is hard-coded to only work with styled-components due to checking the import name here:

stylelint-processor-styled-components/src/utils/styled.js

Lines 6 to 7 in f67b192

	const isStyledImport = node =>
	node.type === 'ImportDeclaration' && node.source.value === 'styled-components'

We should add an option (importName? library? package?) to allow users to say that the name of the imported package is different so it can be used with styled-jss, emotion, etc.

It'd be a shame if all those libraries had to build their own processor, when really they only have to change that one string and it'd work. I don't think we should promise any particular level of support, since all those libraries have slightly diverging APIs, but having basic support would be prettttttty great. (and also awesome to get out with v1.0)

[mxstbr]

Ohh I just now realised that this'll already work as we only check for styled.x calls! This code lints:

import styled from 'emotion';

const Div = styled.div`
  color: blue;
`

It only fails if you use a differing import name, that's what that check is for!

import emotion from 'emotion';

const Div = emotion.div`
  color: blue;
`