Misc changes to tree printing

[jlumpe]

• Added/improved docstrings for printing-related functions and types.
• Add repr_node() function, similar to repr_tree().
• Add :limit => true to IOContext used in default printnode() implementation.
• Default value for charset argument to print_tree() is now defined in the DEFAULT_CHARSETconstant/global rather than
  being returned from the no-argument constructor.
• Defined ASCII_CHARSET constant for TreeCharSet using only ASCII characters.
• Improvements to printing truncation test.

[codecov-io]

Codecov Report

Merging #62 (c6a5106) into master (b5263bd) will decrease coverage by 0.81%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
- Coverage   66.11%   65.30%   -0.82%     
==========================================
  Files           7        7              
  Lines         363      366       +3     
==========================================
- Hits          240      239       -1     
- Misses        123      127       +4     

Impacted Files	Coverage Δ
src/printing.jl	88.23% <28.57%> (-7.60%)	⬇️

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