Add a third option for handling non-existent theme files

[hailwood]

This option assumes that the files do infact exist at that path. This is great for e.g. route asset management (say I have /assets/file.css that actually invokes my AssetManagementController) Or (and this one is my actual use case) you're using webpack-dev-server and so the files are stored and served in memory via a proxy.

If the asset_not_found action is 'ASSUME_EXISTS' then it will return the path relative to the active themes assetPath.

[codecov-io]

Current coverage is 42.50%

Merging #31 into master will decrease coverage by 0.63%

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[hailwood]

The other two cases aren't covered anyway heh.