Add segments config + --renderSegments flag

Named segments can be defined in `hugo.toml`.

* Eeach segment consists of zero or more `exclude` filters and zero or more `include` filters.
* Eeach filter consists of one or more field Glob matchers.
* Eeach filter in a section (`exclude` or `include`) is ORed together, each matcher in a filter is ANDed together.

The current list of fields that can be filtered are:

* path as defined in https://gohugo.io/methods/page/path/
* kind
* lang
* output (output format, e.g. html).

It is recommended to put coarse grained filters (e.g. for language and output format) in the excludes section, e.g.:

```toml
[segments.segment1]
  [[segments.segment1.excludes]]
    lang = "n*"
  [[segments.segment1.excludes]]
    no     = "en"
    output = "rss"
  [[segments.segment1.includes]]
    term = "{home,term,taxonomy}"
  [[segments.segment1.includes]]
    path = "{/docs,/docs/**}"
```

By default, Hugo will render all segments, but you can enable filters by setting the `renderSegments` option or `--renderSegments` flag, e.g:

```
hugo --renderSegments segment1,segment2
```

For segment `segment1` in the configuration above, this will:

* Skip rendering of all languages matching `n*`, e.g. `no`.
* Skip rendering of the output format `rss` for the `en` language.
* It will render all pages of kind `home`, `term` or `taxonomy`
* It will render the `/docs` section and all pages below.

Fixes #10106

commands/commandeer.go

@@ -521,6 +521,7 @@ func applyLocalFlagsBuildConfig(cmd *cobra.Command, r *rootCommand) {
 	cmd.Flags().StringP("cacheDir", "", "", "filesystem path to cache directory")
 	_ = cmd.Flags().SetAnnotation("cacheDir", cobra.BashCompSubdirsInDir, []string{})
 	cmd.Flags().StringP("contentDir", "c", "", "filesystem path to content directory")
+	cmd.Flags().StringSliceP("renderSegments", "", []string{}, "named segments to render (configured in the segments config)")
 	_ = cmd.Flags().SetAnnotation("theme", cobra.BashCompSubdirsInDir, []string{"themes"})
 }