Make go_list_type configurable for each command independently

[fatih]

Currently this setting is global. However we could make it better by giving the user the ability what they want to use for each command. So instead of

let g:go_list_type = "locationlist"

we could have:

let g:go_list_type_commands = {
  "GoFmt": "locationlist",
  "GoBuild": "quickfix"
}

This would override the command's default setting for the list.

[bhcleek]

(partly cross posted from #1365)
I like the idea of making the list type configurable for each command.

Having said that, though, the defaults for each command should still be consistent. In light of #1365, what should be the criteria for deciding the default list type for each command? Making GoFmt use the quickfix list deviates from the original criteria we used; I wonder if there's an implication here that other commands that currently use the location list should instead be using the quickfix list: if GoFmt's default wasn't correct, then are there other commands that are also using the wrong list type for their default? How can we consistently evaluate that?

For reference, #700 introduced the use of both list types and was completed to fix #696.

Use cases are always helpful, and here's one that seems to be a counter point to #1365:
I thought of a use case where using the quickfix list for GoFmt may be less than ideal for most users:

1. :GoBuild
2. Many errors across several files are shown in the quickfix list
3. :cfirst to go to the first error
4. edit to fix error, but introduce a problem for GoFmt
5. :w (assuming g:go_fmt_autosave is set)
6. quickfix window that contained compiler errors is now replaced with errors from GoFmt

As a user, I would find that surprising. What I'd want to happen is to be able to correct the formatting problem and then do :cn to go to the next compiler error, but now I have to do :GoBuild instead to repopulate the quickfix list with compiler errors.

[fatih]

We can go back for gofmt for location list. I think we just need a comment so next time I don't accidentally change it. If you want to work on it feel free to start. I know that you already worked on this behavior.

So I think the following should apply in order:

1. A default for all commands individually, locationlist for :GoFmt, quickfix for :GoBuild etc...
2. A setting to change all settings globally (so for example someone just can use quickfix).
3. A setting that overrides individual commands (this issue).

Do you think this is clear and is worth tackling ?

[bhcleek]

SGTM

I've already started the work and should be able to finish it up soon. I was considering keeping just the single configuration value, g:go_list_type and allowing it to support both maps and a string. If it's a string, then it would apply to all commands, but if it's a map, then it would each command be specified to deviate from that command's default. What do you think?

[fatih]

I'm usually against dynamic types as it's make it hard to refactor :/ I know that Vim itself encourages it in some places. I think having g:go_list_type just for global setting for all commands and introduce a new g:go_list_type_commands would be better.