expose an API, not just the app

[nsajko]

An app can not load a Julia package. At least not conveniently. Suppose someone wanted to measure the error for logit from LogExpFunctions.jl, currently they would need to redefine logit when calling the app on the command line.

Thus it would be preferable to expose an actual API.

[nsajko]

In fact, I suppose there is not much sense in having an app in the first place for this package.