Add safeHtml and replaceSubstitutionTransformer

[albkn]

This adds safeHtml template tag and replaceSubstitutionTransformer

The safeHtml tag is really similar to html, but it does HTML escaping for strings coming from expression interpolations (strings coming from ${ ... }). The idea is that this new tag when combined with the html tag can become a lightweight HTML templating system that is safe from XSS attacks.

Example:

const fruits = ['apple', 'orange', '<script>alert("bad fruit")</script>']
const template = html`
  <ul>
    ${fruits.map(fruit => safeHtml`<li>${fruit}</li>`)}
  </ul>
`;

The replaceSubstitutionTransformer is similar to the replaceResultTransformer but replaces the results of substitutions instead of the end result. This is the transformer that does the regex replacement of characters needed in the safeHtml tag.

[codecov-io]

Current coverage is 100%

Merging #42 into master will not change coverage

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[zspecza]

This is great - thanks for the amazing contribution @morcerf 👍