Allow min_width of a block in i3bar to be a string

Allow min_width of a block in i3bar to be a string

With this change, min_width can either be an integer (as usual), or a
string. In the latter case, the width of the text given by min_width
determines the minimum width of the block. This way one does not have to
figure out a minimum width by trial and error, only to do it again every
time the font is changed.