A way to configure the remote where the tags are pushed to.

I tried numerous times to push tags with gitui.
It appears to not respect local repo git config
```
[remote]
pushDefault = writableOrigin
```
After setting `pushDefault` to the name of `writableOrigin`, `git push --tags` appears to work fine, while the
gitui continues to complain `Bad credentials` and seems to print messages about trying to access `readOnlyOrigin` in its log
with `gitui -l`.

As a work around, renaming `writableOrigin` -> `origin` and `origin` -> `readOnlyOrigin` appears to fix `Bad Credentials` messages from `gitui`.

As a proposed improvement for gitui, which is awesome, and not a way to blame anyone:
1. error messages can be improved, so that more specific cause of error is printed instead of `Bad Credentials`
2. make origin where the tags get pushed to configurable.