page: Ability to specify build tasks per page

Implements: https://todo.sr.ht/~adnano/kiln/15

config.toml

@@ -4,6 +4,7 @@ title = "Example website"
 "/" = "/{{ .Date.Format `2006/01/02` }}/{{ path.Base .Path }}/"
 
 [[tasks]]
+name = "Gemini"
 url = "gemini://example.com"
 input = [".gmi"]
 output = ".gmi"

docs/kiln.1.scd

@@ -120,6 +120,22 @@ The following keys are supported:
 	---
 	```
 
+*outputs*
+	Optionally specifies a list of tasks that can build this page. Defaults to
+	all tasks.
+
+	Example:
+
+	```
+	---
+	outputs: ["Gemini", "HTTPS"]
+	---
+
+	---
+	outputs: [] # Excluded from all tasks
+	---
+	```
+
 *params*
 	Specifies extra parameters to be provided to templates.
 
@@ -211,6 +227,19 @@ Tasks can be specified in the [[tasks]] array of tables.
 
 The following configuration options are supported per task:
 
+*name*
+	An optional name for the task.
+
+	Example:
+
+	```
+	[[tasks]]
+	name = "Gemini"
+
+	[[tasks]]
+	name = "HTML export"
+	```
+
 *input*
 	A list of input file extensions. Files in the content directory with a
 	matching extension will be processed.

page.go

@@ -18,10 +18,11 @@ import (
 
 // Page represents a page.
 type Page struct {
-	Title    string
+	Title    string                 `yaml:"title"`
-	Date     time.Time
+	Date     time.Time              `yaml:"date"`
-	Weight   int
+	Weight   int                    `yaml:"weight"`
-	Params   map[string]interface{}
+	Outputs  []string               `yaml:"outputs"`
+	Params   map[string]interface{} `yaml:"params"`
 	Path     string                 `yaml:"-"`
 	FilePath string                 `yaml:"-"`
 	URL      string                 `yaml:"-"`
@@ -113,6 +114,21 @@ func (p *Page) _read(fsys fs.FS, path string, task *Task, cfg *Site) error {
 			if len(frontmatter) != 0 {
 				if err := yaml.Unmarshal(frontmatter, page); err != nil {
 					log.Printf("failed to parse frontmatter for %q: %v", path, err)
+				} else if page.Outputs != nil {
+					// Enforce page outputs
+					if len(page.Outputs) == 0 || task.Name == "" {
+						continue
+					}
+					found := false
+					for _, output := range page.Outputs {
+						if output == task.Name {
+							found = true
+							break
+						}
+					}
+					if !found {
+						continue
+					}
 				}
 
 				// Trim leading newlines from content

site.go

@@ -25,6 +25,7 @@ type Site struct {
 
 // Task represents a site build task.
 type Task struct {
+	Name        string            `toml:"name"`
 	Input       []string          `toml:"input"`
 	OutputExt   string            `toml:"output"`
 	TemplateExt string            `toml:"template"`