feat: Add csv output functionality

[jandrovins]

I know you could transpose the -m output to make it CSV-compatible, but I think this is more convenient. Let me know what you think.

[jandrovins]

I should comment on the changes done and the functionality added.

In terms of functionality, having direct csv output makes it easier to have output useful for spreadsheets. Sample of output:

$ bigotes -c -w -t 1 ls -lr
median,mean,min,q1,q3,max,samples,wall,mad,stdev,skewness,kurtosis,far,rfar,rmad,rstdev,sem,rsem,sw_pvalue,sw_normal,runmode,command,arg1
2.583332e-03,2.703544e-03,2.143072e-03,2.513124e-03,2.712377e-03,8.434378e-03,370,1.000311e+00,1.333704e-04,5.462736e-04,7.162707e+00,6.288460e+01,19,5.135135e+00,5.162729e+00,2.020583e+01,2.839941e-05,2.058884e+00,2.764281e-33,0,exec,ls,-lr

In terms of changes done in the code: Instead of directly printing the output, bigotes now creates a string buffer to where the output is stored before printing. For when using csv output, we need 2 buffers to store in the one hand the header row and in the other the values per column, as to print before the header row.