Add convenience methods to for constructing IonObjectMapper.Builder

[mcliedtke]

This was referenced in #246 and it appears that it may have been resolved by accident.

In any case, this serves to just add some convenience to the creation of an IonObjectMapper:

• Explicitly creating a builder for text or binary writers
• Allow a specified IonSystem at builder construction time
• Add convenience methods for setting IonGenerator.Feature or IonParser.Feature

These convenience methods respect the current defaults:

• By default textual writers are enabled
• By default, a standard IonSystem is used (IonSystemBuilder.standard().build())

I've made this change to the 2.12 branch but this isn't urgent and can be moved to 2.13, though it should be backwards compatible

[cowtowncoder]

Agreed, this is a good improvement. Just one question: should this go against 2.13? Ideally since it's API addition, it should; although 2.12.2 is bit special with quite a few non-compliant additions (to resolve existing problems).

[mcliedtke]

I am not particularly strongly opinionated one way or the other. I pretty much had the same thought that 2.12.2 is already a bit special and this doesn't seem all that different to include. But it is also purely just added convenience so there is no rush and could easily wait until 2.13

[cowtowncoder]

@mcliedtke Ok. With that, I think I might as well just merge it as-is :)

[cowtowncoder]

Ah. Was about to comment on inability to set IonParser/Generator features... until I realized they will work, just need to do it bit different. So that's fine. :)