Add option to output graph dot text files

[aeflores]

This PR adds the capability of producing the precedence and SCC graphs in text format (in addition to the svg format).
This format is more amenable for downstream analyses and processing.

[codecov]

Codecov Report

Merging #2092 (0bca50d) into master (8ecfe4e) will decrease coverage by 0.04%.
The diff coverage is 7.69%.

@@            Coverage Diff             @@
##           master    #2092      +/-   ##
==========================================
- Coverage   76.59%   76.55%   -0.05%     
==========================================
  Files         448      448              
  Lines       28025    28047      +22     
==========================================
+ Hits        21467    21472       +5     
- Misses       6558     6575      +17     

Impacted Files	Coverage Δ
src/ast/analysis/PrecedenceGraph.cpp	35.13% <0.00%> (-9.70%)	⬇️
src/ast/analysis/PrecedenceGraph.h	100.00% <ø> (ø)
src/ast/analysis/SCCGraph.cpp	69.73% <0.00%> (-8.21%)	⬇️
src/ast/analysis/SCCGraph.h	100.00% <ø> (ø)
src/main.cpp	67.97% <33.33%> (-0.64%)	⬇️
src/interpreter/Engine.cpp	90.56% <0.00%> (-0.13%)	⬇️
src/include/souffle/datastructure/LambdaBTree.h	43.51% <0.00%> (+1.52%)	⬆️
src/include/souffle/datastructure/UnionFind.h	97.80% <0.00%> (+2.19%)	⬆️

[b-scholz]

Thanks for the contribution!