Added "bar.space" parameter to set space between bars in bar charts

[kt3k]

This PR supercedes #1815.

#1815 was originally sent to dev branch. I cherry-picked f3fd649 to master branch and it seems working as well.

samples/chart_bar_space.html

c3.generate({
  data: {
    columns: [
      ['data1', 30, 200, 100],
      ['data2', 50, 20, 10],
      ['data3', 150, 120, 110]
    ],
    type: 'bar'
  },
  bar: {
    space: 0.25
  }
});

---

close #1815
close #1328
close #1327

[codecov-io]

Codecov Report

Merging #2146 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2146      +/-   ##
==========================================
+ Coverage   73.63%   73.64%   +<.01%     
==========================================
  Files          52       52              
  Lines        4131     4132       +1     
==========================================
+ Hits         3042     3043       +1     
  Misses       1089     1089

Impacted Files	Coverage Δ
src/config.js	95.83% <ø> (ø)	⬆️
src/shape.bar.js	90.27% <100%> (+0.13%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 49306eb...9f309fe. Read the comment docs.