Proposal: Add templating support for CLI flags list/list-all

[trulede]

This is a proposal (code is finished) adding template support for CLI commands --list / --list-all. The implementation adds an optional flag (--template) which is used to specify a template file(s). When specified, the template files(s) are loaded and rendered. The templating mechanism makes use of slim-sprig functions.

Rational is to allow generation of documentation from Taskfiles, or provide custom formatting for these commands.

fixes #1695
fixes #2261
related #1916
related #931 (perhaps)
related #2091

If the proposal is welcome, then at least the following might also be of interest for this PR:

• Documentation with examples.
• Application to the --summary command.
• Support template definition in the taskrc file.
• Printing debug info during task execution, using some combination of previous items.
• Additional tests.
• Base templating on JSON data from (i.e. e.ToEditorOutput()). Perhaps?

Example

mddoc.template

# Taskfile
Available tasks for this project:
{{ range . }}
## {{.Task}}
{{.Desc}}
{{if len .Aliases}}
### Aliases
{{range .Aliases}}- {{.}}
{{end}}{{end}}{{end}}

Console Ouput

$ task --list-all --template mddoc.template
# Taskfile
Available tasks for this project:

## clean
Cleans temp files and folders

### Aliases
- clear

## default


## format
Runs golangci-lint and formats any Go files

### Aliases
- fmt
- f

## generate
Runs all generate tasks

### Aliases
- gen
- g