histogram: Support aligning bars at the right edge of bins

In `histogram`, by default, bars are aligned at the left edge of the bins. With `-F` option, bars are centered in the bins.

The behavior can be confirmed below:
```
gmt begin map png
gmt histogram @v3206_06.txt -R-7000/1000/0/2000 -T250 -W0.25p -Baf250 -Gblue
gmt histogram @v3206_06.txt -R-7000/1000/0/2000 -T250 -W0.25p -Baf250 -Gblue -F -Xw+1c
gmt end show
```

What's missing is aligning the bars at the right edge of the bins. For comparison, matplotlib's `hist` method provides the `algin` parameter which can be "left", "right", and "mid".

It would be great if GMT's histogram can also support it. For backward compatibility, I expect the following syntax:

- `-Fc` or `-F`: centering
- `-Fl`: left alignment (the default without -F)
- `-Fr`: right alignment