Only output used prefixes

[Aklakan]

Output of all formats that support prefixes cause all of prefix.cc prefixes to be written out. This makes output so ugly its not funny.

• By default, output should be deferred until a certain amount of data has been seen on which basis the set of used prefixes is determined. This is likely to work for 80-99% of the use cases. The amount of data to consider should be configurable.
• To capture the remaining corner cases, there should be an option to provide the output prefixes directly

completed deferred output support on:

• [x] sparql integrate
• [ ] named graph streams (ngs)
• [n/a] sparql binding streams (sbs)

[Aklakan]

Deferred output with used prefix analysis is now implemented. Need to add the defer-output flag to all commands.

[Aklakan]

There should be a --out-prefixes option where a file with prefix declarations can be specified.