cmd/scriggo: implement 'scriggo run' to run a template file

[gazerro]

This proposal want to implement a scriggo subcommand to render a single template file as proposed in discussion #875:

scriggo run [ -o output ] [ -root dir ] [ -const name=value ] [ -format format ] [ -metrics ] [-S n] path

It runs the file with the given path, relative to the file's directory, and write the resulting code to the standard output. The file format is determined by the file extension.

The -o flag writes the resulting code to the named output file or directory.

The -root flag sets the root directory to the named directory instead of the file's directory.

The -const flag runs the template file with a global constant with the given name and value. name should be a Go identifier and value should be a string literal, a number literal, true or false. There can be multiple name=value pairs.

The -format flag forces render to use the named file format.

The -metrics flag prints metrics about execution time.

The -S flag prints the assembly code of the executed file and n determines the maximum length, in runes, of disassembled Text instructions

n > 0: at most n runes; leading and trailing white space are removed
n == 0: no text
n < 0: all text

Examples:

$ scriggo run index.html

$ scriggo run -const 'version=1.12 title="The ancient art of tea"' index.md

$ scriggo run -root . docs/article.html

$ scriggo run -format Markdown index

$ scriggo run -o ./public ./sources/index.html