made most dependencies optional with Requires.jl

[lungben]

Putting all packages for importing different file formats as direct dependencies is quite convenient for the user, but has serious drawbacks:

1. The time for importing TableIO is very long (ca. 30s)
2. The installation of TableIO is very heavy-weight and includes many packages the user probably does not need
3. There may be issues due to incompatible dependences

Here, I am making all dependencies except CSV.jl and DataFrames.jl optional using Revise.jl. This makes the installation more light-weight and importing much faster (4s after precompilation for me).

ToDo: document how to use the optional dependencies

[codecov-commenter]

Codecov Report

Merging #9 into master will increase coverage by 4.59%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master        #9      +/-   ##
===========================================
+ Coverage   95.40%   100.00%   +4.59%     
===========================================
  Files           3         8       +5     
  Lines          87        89       +2     
===========================================
+ Hits           83        89       +6     
+ Misses          4         0       -4     

Impacted Files	Coverage Δ
src/TableIO.jl	100.00% <100.00%> (ø)
src/jdf.jl	100.00% <100.00%> (ø)
src/parquet.jl	100.00% <100.00%> (ø)
src/postgresql.jl	100.00% <100.00%> (ø)
src/sqlite.jl	100.00% <100.00%> (ø)
src/stat_files.jl	100.00% <100.00%> (ø)
src/xlsx.jl	100.00% <100.00%> (ø)
src/zipped_csv.jl	100.00% <100.00%> (ø)
... and 5 more

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