Support Zeplin's new Spacing feature

[freak4pc]

This allows leveraging Zeplin's new Spacing feature presented last week here:
https://blog.zeplin.io/introducing-spacing-and-rem-unit-5f06d10df5e3

You can use spacing in a Prism template as follows:

enum Spacing {
    {{% FOR spacing %}}
    static let {{% spacing.identity.camelcase %}} = {{% spacing.value %}}
    {{% END spacing %}}
}

<!-- This file was generated using Prism -->
<?xml version="1.0" encoding="utf-8"?>
<resources>
    {{% FOR spacing %}}
    <dimen name="{{% spacing.identity.snakecase %}}">{{% spacing.value %}}dp</dimen>
    {{% END spacing %}}
</resources>

[codecov]

Codecov Report

Merging #17 into master will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   98.52%   98.67%   +0.15%     
==========================================
  Files          19       19              
  Lines        1014     1131     +117     
==========================================
+ Hits          999     1116     +117     
  Misses         15       15              

Impacted Files	Coverage Δ
Sources/ZeplinAPI/Models/Color.swift	100.00% <ø> (ø)
Sources/PrismCore/Models/AssetIdentity.swift	100.00% <100.00%> (ø)
Sources/PrismCore/Models/ProjectAssets.swift	100.00% <100.00%> (ø)
Sources/PrismCore/Prism.swift	96.36% <100.00%> (+0.85%)	⬆️
...rismCore/TemplateParser/TemplateParser+Token.swift	100.00% <100.00%> (ø)
...TemplateParser/TemplateParser+Transformation.swift	100.00% <100.00%> (ø)
...rces/PrismCore/TemplateParser/TemplateParser.swift	100.00% <100.00%> (ø)
Sources/ZeplinAPI/ZeplinAPI.swift	100.00% <100.00%> (ø)
Tests/TemplateParserSpec.swift	100.00% <100.00%> (ø)

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