Add GetFieldBlock methods to remove fields from final result set

[mrtl-srn]

Example :

const squel = require('squel');

const sql = squel.select();

sql.field('field1')
   .field('field2', 'alias2')
   .field('field1', 'alias1')
   .from('table1');

/* SELECT field1, field2 as alias2, field1 as alias1 FROM table1; */

sql.removeField('alias1')
   .removeField('field2');

/* SELECT field1 FROM table1; */

sql.removeAllFields();

/* SELECT * FROM table1; */

[codecov]

Codecov Report

Merging #344 into master will decrease coverage by 0.23%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #344      +/-   ##
==========================================
- Coverage   81.31%   81.07%   -0.24%     
==========================================
  Files           2        2              
  Lines        2863     2885      +22     
  Branches      635      639       +4     
==========================================
+ Hits         2328     2339      +11     
- Misses        535      546      +11

Impacted Files	Coverage Δ
dist/squel.js	71.8% <0%> (-0.49%)	⬇️
dist/squel-basic.js	93.44% <100%> (+0.05%)	⬆️

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