Let logarithimc plots accept negative values

**Description of the desired feature**

It is common now to have parameters varying from negative to positive with a large spread. To plot that, logarithmic or exponential plots may be good choices. However, GMT currently accepts positive ranges only since it only takes the u'=alog10(u)+b.

Could if judge the sign of u before taking the logarithmic transformation, and then append the sign back? Just like the symlog module in other plotting software.
If so, the plotting module could easily handle the rest since it originally accepts both negative and positive values.