Add --indent-string option to libcst.tool print

[amyreese]

Summary

Allows passing a custom indent string, like ". ", for easier visual
parsing of the resulting tree.

Test Plan

(.venv) jreese@butterfree ~/workspace/usort imr± » cat > hello.py
print("hello world!")

(.venv) jreese@butterfree ~/workspace/usort imr± » python -m libcst.tool print --show-whitespace --show-syntax --indent-string ".   " hello.py
Module(
.   body=[
.   .   SimpleStatementLine(
.   .   .   body=[
.   .   .   .   Expr(
.   .   .   .   .   value=Call(
.   .   .   .   .   .   func=Name(
.   .   .   .   .   .   .   value='print',
.   .   .   .   .   .   ),
.   .   .   .   .   .   args=[
.   .   .   .   .   .   .   Arg(
.   .   .   .   .   .   .   .   value=SimpleString(
.   .   .   .   .   .   .   .   .   value='"hello world!"',
.   .   .   .   .   .   .   .   ),
.   .   .   .   .   .   .   ),
.   .   .   .   .   .   ],
.   .   .   .   .   ),
.   .   .   .   ),
.   .   .   ],
.   .   ),
.   ],
.   footer=[
.   .   EmptyLine(),
.   ],
)

[amyreese]

Related: would anyone be interested in changing the default indent string to something more useful than " "?

[codecov-commenter]

Codecov Report

Merging #525 (2585671) into main (39607ed) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #525      +/-   ##
==========================================
- Coverage   94.77%   94.77%   -0.01%     
==========================================
  Files         235      235              
  Lines       23136    23137       +1     
==========================================
  Hits        21928    21928              
- Misses       1208     1209       +1     

Impacted Files	Coverage Δ
libcst/tool.py	33.22% <0.00%> (-0.11%)	⬇️

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[lisroach]

Good idea, I am +1 on a more readable default

[zsol]

FWIW I agree, sorry for not getting to this in time :)