Support for table column spans

[Eugenij-W]

to: #29

I implement Wiky-style cell-span extension.

Simple example:

| ta | tb | tc |
|----|----|----|
| a  | b  | c  |
|2> cspan | c  |
| a  | b  | c  |

convert into:

Hard exaple:

| day |6>               miner-a                 |6>              miner-b                  |
|----:|:----:|:----:|:----:|:----:|:----:|:----:|:----:|:----:|:----:|:----:|:----:|:----:|
|    1|6>                 121                   |6>                 281                   |
|    2|6>                 190                   |6>                 119                   |
|    3|6>                 246                   |6>                 199                   |
|     |2>  summary: |4>   557                   |2>  summary: |4>   599                   |
|     |6>          added a video card           |6>           added a video card          |
|     |3>    miner-a-1     |3>    miner-a-2     |3>     miner-b-1    |3>     miner-b-2    |
|    4|3>       159        |3>       165        |3>        160       |3>        287       |
|    5|3>       293        |3>       123        |3>        163       |3>        157       |
|    6|3>       284        |3>       221        |3>        189       |3>        139       |
|     |2>  summary: |4>   1245                  |2>  summary: |4>   1095                  |
|     |6>          added a video card           |6>         added a video card            |
|     |2> miner-a-1 |2> miner-a-2 |2> miner-a-3 |2> miner-b-1 |2> miner-b-2 |2> miner-b-3 |
|    7|2>    134    |2>    251    |2>    287    |2>    183    |2>    199    |2>    274    |
|    8|2>    247    |2>    194    |2>    150    |2>    227    |2>    186    |2>    108    |
|    9|2>    268    |2>    203    |2>    106    |2>    248    |2>    160    |2>    161    |
|     |2>  summary: |4>   1840                  |2>  summary: |4>   1746                  |
|     |6>          2 stream per kernel          |6>          2 stream per kernel          |
|     | a1-1 | a1-2 | a2-1 | a2-2 | a3-1 | a3-2 | b1-1 | b1-2 | b2-1 | b2-2 | b3-1 | b3-2 |
|   10|  200 |  227 |  249 |  142 |  238 |  242 |  107 |  182 |  141 |  219 |  229 |  143 |
|   11|  196 |  161 |  121 |  209 |  133 |  137 |  105 |  205 |  240 |  209 |  199 |  131 |
|   12|  231 |  171 |  212 |  117 |  219 |  177 |  158 |  189 |  202 |  187 |  121 |  106 |
|     |2>  summary: |4>   3382                  |2>  summary: |4>   3073                  |

convert into:

[codecov]

Codecov Report

Merging #147 (27474ef) into master (2b6ebdf) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #147      +/-   ##
==========================================
+ Coverage   94.45%   94.48%   +0.03%     
==========================================
  Files           3        3              
  Lines        3066     3084      +18     
==========================================
+ Hits         2896     2914      +18     
  Misses        170      170              

Impacted Files	Coverage Δ
src/md4c-html.c	95.31% <100.00%> (+0.11%)	⬆️
src/md4c.c	94.39% <100.00%> (+0.02%)	⬆️

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