add support for goimports -srcdir in Format

[mhamrle]

"goimports -srcdir" fixes problems with vendoring.

[wuman]

I can confirm that this PR works well.

[fatih]

I was going to add this as this is broken if the user uses an old version of goimports. We need somehow warn the user to update the binary.

[fatih]

I've tested it and it works well. But if you don't update your goimports binary it breaks vim-go. But I think it's ok to move on. This is going to be merged to master and it's not the stale version, so it's ok. Thanks @mhamrle

Also let me put this commit here which introduces this new feature: golang/tools@bf084ef

[mhamrle]

I added check if goimports --help contains -srcdir argument. If not the warning message is displayed and the srcdir arg is not used.

[fatih]

@mhamrle I've thought about it but I think it would slow down. How did you do the check? Can you open another PR so we can talk about it ?

[fatih]

@mhamrle we can speed it up with an intermediate cache value. So we only check once, if it's available we set goimports_updated =1 and don't check it again till the user restarts vim.