Add node attribute bracket syntax

[j6k4m8]

Adds support for "bracket" syntax for node attributes. An attribute like XYZ(ABC) or ABC DEF used to be disallowed because of illegal characters in the attribute name, particularly when using the "dot-attribute" notation:

# broken:
A -> B
A.ABC DEF > 10

The new syntax uses bracket-attribute notation to "escape" these names:

# working:
import dotmotif
from dotmotif.executors.NeuPrintExecutor import NeuPrintExecutor

HOSTNAME = "neuprint.janelia.org"
DATASET = "hemibrain:v1.2.1"
TOKEN = "[YOUR TOKEN HERE]"

motif = dotmotif.Motif("""
A -> B
A['AVLP(R)'] = True
""")

E = NeuPrintExecutor(HOSTNAME, DATASET, TOKEN)

E.find(motif, limit=2)

Fixes #111.

[codecov-commenter]

Codecov Report

Merging #112 (0ec9954) into master (6f30f24) will decrease coverage by 0.11%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
- Coverage   88.44%   88.32%   -0.12%     
==========================================
  Files          25       25              
  Lines        1843     1868      +25     
==========================================
+ Hits         1630     1650      +20     
- Misses        213      218       +5     

Impacted Files	Coverage Δ
dotmotif/executors/test_dm_cypher.py	100.00% <ø> (ø)
setup.py	0.00% <0.00%> (ø)
dotmotif/executors/Neo4jExecutor.py	68.51% <58.33%> (-0.82%)	⬇️
dotmotif/__init__.py	95.34% <100.00%> (ø)
dotmotif/parsers/v2/test_v2_parser.py	100.00% <100.00%> (ø)

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