Simplify Imputor API

[rofinn]

Attempting to resolve the following issues: #66, #57, #51, #50, #44, #43, #31

• [X] Drop Context
• [X] Introduce a separate Threshold type and assert function
• [X] Update old tests cases to use new type
• [X] Add deprecations for old context calls
• [X] Generalize the dimensionality behaviour using a dims keyword similar to the stats functions https://github.com/JuliaLang/Statistics.jl/blob/master/src/Statistics.jl#L164 (using special cased :rows and :cols dims to improve readability)
• [x] ~Drop in-place calls~ Might leave this for now
• [X] Base imputors should dispatch on _impute!(AbstractArray{Union{T, Missing}}, imp)
• [X] Replace dropobs and dropvars with Impute.filter and a dims keyword
• [x] Make Chain not an imputor and have it work on Assertions and Imputors
• [x] ~Add function for checking if a methods support some input data? This might just be helpful for testing?~ Gonna leave this for now.

[rofinn]

Alright, this was bigger than expected, but a lot of the LOC changes are tied to removing the context blocks and moving a couple files around (also adding tests and docs). I've left several issues as TODO items to avoid making this bigger.

@oxinabox Would you mind double checking that I'm using DataDeps.jl as expected?

[rofinn]

Alright, I believe I've either addressed, responded or opened an issue for each comment left. Since there don't seem to be any new comments I'm gonna merge this and get started on the opened issues tomorrow.