Sub command `grep` to help with initial setup.

$ pycalver grep "\[PyCalVer vYYYY0M.BUILD[-RELEASE]\]" README.md
Pattern compiled to '\\[PyCalVer v(?P<year_y>[1-9][0-9]{3})(?P<month>(?:1[0-2]|0[1-9]))\\.(?P<bid>[0-9]+)(?:\\-(?P<tag>(?:preview|final|alpha|beta|post|pre|dev|rc|a|b|c|r)))?\\]'
                                                                                                                                                                                  
Found 1 match for pattern '\[PyCalVer vYYYY0M.BUILD[-RELEASE]\]' in README.md                                                                                                     
                                                                                                                                                                                  
  15: [![Supported Python Versions][pyversions_img]][pyversions_ref]                                                                                                              
  16: [![PyCalVer v202007.1036][version_img]][version_ref]                                                                                                                        
  17: [![PyPI Releases][pypi_img]][pypi_ref]