modify format for array selecting

when selecting in an array a[,1:5,1:10]
tidy_block formats it into a[, 1:5, 1:10]
but the first comma is always easy to be neglected (not beautiful as well)
alternatively, if formatted into a[ , 1:5, 1:10]
it clearly shows that the array has 3 dimensions and what's the conditions for each.

requesting a pull to get familiar with github too..