Plotreg enhancement

[czucca]

Summary of the enhancements:

• Uniformes plotreg to the logic of the other texreg functions, making it plot standard errors by default. ci.force can be used to plot CI in models that provide standard errors by default.
• Changes the plot labels accordingly as requested in #176
• Introduced the confidence interval in the plot label (e.g., 95%) by default. (I personally benefit from this since I constantly forget to put it in the notes).
• Removes inner upper and lower CI to make it simpler to understand for the reader, as requested in #176
• Inserts ci.test as requested in #168
• Inserts every argument that comes together with ci.force
• Optimizes code in several points

After assessing that it is impossible to import matrixreg since it does not provide the right format for plotreg, the function still imports the data from models. I fully tested the new function, but more testing is always good. Apologies for the delay, C.

[codecov[bot]]

Codecov Report

Merging #181 (61a2485) into master (92e3931) will increase coverage by 0.34%. The diff coverage is 76.38%.

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
+ Coverage   40.72%   41.06%   +0.34%     
==========================================
  Files           3        3              
  Lines        6698     6713      +15     
==========================================
+ Hits         2728     2757      +29     
+ Misses       3970     3956      -14     

Impacted Files	Coverage Δ
R/texreg.R	80.10% <76.38%> (+0.78%)	:arrow_up:

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