option --precision is added

[Ev2geny]

I believe this PR fixes issues #65, #92 and partially issue #116.

It fixes it by adding option to effectively ignore dcontext, build by exploring ledger by introducing the following option

  -p {std,raw}, --precision {std,raw}
                        Defines the approach to determine the precision with which  prices are printed (default: std).
                        std - standard beancount approach
                        raw - print the prices with the precision, received from the data source