Slightly tweak the API

[davidanthoff]

Do you think this makes sense?

I mostly care about the final API, i.e. I think it would be good if both

db = Database(filename)

and

db = Database(SQLite.DB(filename))

worked.

I think SourceTables is too opaque of a name for end users.

[codecov-io]

Codecov Report

Merging #5 into master will increase coverage by 0.15%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage    69.5%   69.65%   +0.15%     
==========================================
  Files           8        8              
  Lines         200      201       +1     
==========================================
+ Hits          139      140       +1     
  Misses         61       61

Impacted Files	Coverage Δ
src/QuerySQLite.jl	100% <ø> (ø)	⬆️
src/library.jl	61.11% <ø> (ø)	⬆️
src/source.jl	73.68% <100%> (+1.46%)	⬆️

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[bramtayl]

Ok sounds good