Add omit_columns option to mermaid

[syou6162]

resolves #78

Description

I added --omit_columns option to omit columns from diagrams when the output format is mermaid.

I thought about omitting columns for --target other than mermaid, but since dbml, for example, requires at least one column, I decided to support only --target = mermaid.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2ae1867) 85.60% compared to head (b172040) 85.68%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #77      +/-   ##
==========================================
+ Coverage   85.60%   85.68%   +0.07%     
==========================================
  Files          31       31              
  Lines         910      915       +5     
==========================================
+ Hits          779      784       +5     
  Misses        131      131              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.