Add register_path_accessor to create new options for registering paths

[pjbull]

• Adds register_path_accessor to create custom accessors for objects that implement Path
• Update readme with examples
• Update readme badges after #3
• Add tests for Path of RHS and LHS for joining

Closes #1

[codecov[bot]]

Codecov Report

Merging #4 (9493248) into master (ca1e650) will decrease coverage by 0.2%. The diff coverage is 94.8%.

@@           Coverage Diff            @@
##           master      #4     +/-   ##
========================================
- Coverage    97.6%   97.4%   -0.3%     
========================================
  Files           3       3             
  Lines         127     154     +27     
========================================
+ Hits          124     150     +26     
- Misses          3       4      +1     

Impacted Files	Coverage Δ
pandas_path/accessor.py	94.1% <93.7%> (-0.3%)	:arrow_down:
pandas_path/__init__.py	100.0% <100.0%> (ø)
pandas_path/tests.py	100.0% <100.0%> (ø)