Support basic operations in xml

[RaySuhyunLee]

Support number operators and string concatenation operator(+)

[RaySuhyunLee]

This should be expanded to support every view-related operations inside xml. Refer to #4.

[RaySuhyunLee]

Possible solution: using script interpreter
ex) Python
https://wiki.python.org/jython/JythonMonthly/Articles/September2006/1

[RaySuhyunLee]

Better implement a custom parser by myself. The reasons are:

1. Introducing another specific language may lead to more complicated framework.
2. Using interpreter would increase overhead a lot.

[RaySuhyunLee]

I decided not to implement this feature for the moment. Few reasons are:

1. View should concern at least about processing the data. It should only concern about what to show.
2. Allowing to use operators inside template means leaving possibility that the view can process the data.