Add subDomainValueFormat to format values

[k-nut]

As proposed in #146 the new subDomainValueFormat lets developers pass their own function that they would like to apply to the number before it is displayed in the popup. This way one can for example make sure that the value is formatted in the proper locale.

Is this the right way to do it? It pretty much just copies the formatDate function. If you think that this is ok I will make sure to write some tests as well.

[coveralls]

Coverage decreased (-0.4%) to 77.73% when pulling acca73f8453f9318ddc37114a91fab32a30c2adf on k-nut:add-option-to-format-values into 5b929fc85479592b96b9fca5bd700d058859af08 on kamisama:master.

[k-nut]

any feedback on this?

[nivhanin]

+1 !!