Use 'D' symbol in format to specify minimal length of fractional part

This request address issues #45 and #63 (as far as I understand this issues duplicate each other). Now it's possible to specify minimal length of fractional part using 'D' symbol. At the same time 'd' symbol used to specify maximal length as it works in current version.

Here some examples how format works. Assume that value is `1.23`:
- `( ddd).d` became `1.2`
- `( ddd).D` became `1.2`
- `( ddd).ddd` became `1.23`
- `( ddd).DDD` became `1.230`
- `( ddd).Ddd` became `1.23`
- `( ddd).DDd` became `1.23`
- `( ddd).dD` will raise exception

Look for live sample in `trailing-zero.html` file in `test` folder