Add support to use manual DAP configuration

[HeavyPunk]

Hi!
Thank you for awesome plugin!

I reported a motivation for this changes in PR #216 but will describe a main line at here:
At this moment test debugging only possible with neovim-dap-go but I already had a DAP configuration for golang and no need to install neovim-dap-go.
In PR split a DAP strategy for 2 possible ways: using neovim-dap-go (a default behavior) or using manual DAP configuration from config

If you have any question, you are always welcome!
Thank you!

[fredrikaverpil]

Closing in favour for PR: #216