feat: `--keyword` / `--keyword-plural` for `xgotext`

[decentral1se]

In xgettext-go you have several additional flags like this invocation (pulled from the incus project):

xgettext-go -o po/$(DOMAIN).pot --add-comments-tag=TRANSLATORS: --sort-output --package-name=$(DOMAIN) --msgid-bugs-address=lxc-devel@lists.linuxcontainers.org --keyword=i18n.G --keyword-plural=i18n.NG cmd/incus/*.go shared/cliconfig/*.go

I'd like to add the --keyword=... / --keyword-plural=... to xgotext which I personally need for a project.

Just creating this in case there are objections, PR incoming when I get time.

[decentral1se]

@leonelquinteros would you be able to explain the significance of the the int values in this part? It seems like I just need to inject my --keyword params into this map at some point 🤔

gotext/cli/xgotext/parser/gofile.go

Lines 28 to 38 in 270f280

	// list of supported getter
	var gotextGetter = map[string]GetterDef{
	"Get": {0, -1, -1, -1},
	"GetN": {0, 1, -1, -1},
	"GetD": {1, -1, -1, 0},
	"GetND": {1, 2, -1, 0},
	"GetC": {0, -1, 1, -1},
	"GetNC": {0, 1, 3, -1},
	"GetDC": {1, -1, 2, 0},
	"GetNDC": {1, 2, 4, 0},
	}