[Feature] add an option to axis.label.style.textAlign to make all the labels align in the center of the whole axis component

[da730]

What problem does this feature solve?

While label.style.textAlign support 'center' currently, the baseline of the text is not in the middle of the whole axis component:

We need a new option to make the baseline of the align in the middle of the axis (0.5*width of the axis):

What does the proposed API look like?

label.style.textAlign='axisCenter'

[kkxxkk2019]

@visactor/vchart@1.3.0-beta.3 support it