Draft: Accepting already open and limited IO types

[JonasIsensee]

This adds a method jldopen(::IO, mode; kwargs...)
that should take in already opened IOs and figure out what to do with them.

Currently, this PR implements a ReadOnlyBuffer that can wrap another IO object which only supports reading but not seeking. Therefore it reads linearly and buffers everything in between.

• figure out what to implement for reading
• figure out what to implement for writing
• How to handle a capable external stream (light-weight wrapper)
• How to figure out what an IO object can do (how to wrap it or error)
• Do something creative with the filename field in JLDFile

Referencing
#346
#233
#304

[codecov]

Codecov Report

Attention: Patch coverage is 94.45585% with 27 lines in your changes missing coverage. Please review.

Project coverage is 85.45%. Comparing base (bc774d3) to head (1557580).
Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
src/io/bufferedio.jl	78.78%	14 Missing ⚠️
src/io/dataio.jl	97.45%	4 Missing ⚠️
src/JLD2.jl	95.38%	3 Missing ⚠️
src/headermessages.jl	60.00%	2 Missing ⚠️
src/io/io_wrappers.jl	97.75%	2 Missing ⚠️
src/data/specialcased_types.jl	50.00%	1 Missing ⚠️
src/types.jl	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #535      +/-   ##
==========================================
+ Coverage   85.21%   85.45%   +0.23%     
==========================================
  Files          36       37       +1     
  Lines        4391     4406      +15     
==========================================
+ Hits         3742     3765      +23     
+ Misses        649      641       -8     

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