Drop (groups of) variables, option to display like fixed effects

[greimel]

fixes #12 and #17

Current status:

omit = ["Sepal" => r"^SepalWidth", "(Intercept)"]

RegressionTables.regtable(rr5,rr6,lm1, lm2, lm3; omit)

----------------------------------------------------------------------
                   SepalWidth                 SepalLength             
                   ----------   --------------------------------------
                          (1)       (2)       (3)        (4)       (5)
----------------------------------------------------------------------
SepalLength            -0.313                                         
                      (0.239)                                         
PetalLength           1.048**                                         
                      (0.362)                                         
PetalWidth                                          0.972***   1.291**
                                                     (0.052)   (0.474)
----------------------------------------------------------------------
Sepal                           kind of   kind of    kind of       Yes
----------------------------------------------------------------------
Species                   Yes                                         
isSmall                             Yes                               
Species * isWide                    Yes                               
----------------------------------------------------------------------
Estimator                  IV       OLS       OLS        OLS       OLS
----------------------------------------------------------------------
N                         150       149       150        150       150
R2                      0.080     0.726     0.014      0.707     0.708
----------------------------------------------------------------------

[codecov]

Codecov Report

Attention: Patch coverage is 48.38710% with 32 lines in your changes missing coverage. Please review.

Project coverage is 89.62%. Comparing base (879a780) to head (4d0e81b).
Report is 158 commits behind head on master.

Files with missing lines	Patch %	Lines
src/regtable.jl	48.38%	32 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
- Coverage   95.31%   89.62%   -5.69%     
==========================================
  Files          11       11              
  Lines         512      530      +18     
==========================================
- Hits          488      475      -13     
- Misses         24       55      +31     

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