Implement omision of zeroed-fields with + flag

[2opremio]

Fixes #59

This PR binds the '+' formatting flag to omitting empty (zero-valued) struct fields.

[2opremio]

@kr friendly ping :)

[2opremio]

@kr any chance of taking a look at this anytime soon?

[kr]

I like the idea of omitting zero fields, since that's pretty common in Go source code to begin with.

But maybe it should not be an option. I'm not sure retaining the ability to print zero fields carries its weight.

What do you think?

[kr]

Or, if it should be an option, maybe the notation should be % +v (instead of % +#v). I think that would be a little closer to the stdlib behavior of %+v and %#v.

[2opremio]

No strong opinions here. I can implement whichever option you prefer. just let me know :)

[2opremio]

I have given a Go to % +v (instead of % +#v) and I think it's going to be confusing for the end user (adding a single sign, as already found in the PR, to change the behavior is much more intuitive).

@kr what do you say? It's already been a month since I submitted the PR and I would like it to move forward. Thanks!

[2opremio]

@kr friendly ping

[2opremio]

@kr friendly ping

[nikolay-turpitko]

+1 for this feature. Regarding format, I'd omit zero values by default (with %# v) and show them with something like %# 0v (zero padding flag). Or vise versa, if you afraid to introduce breaking change. It both closer to existing format and easier to remember. WDYT?

[2opremio]

@kr friendly ping