Add more configuration options to confique::FormatOptions

[mtfwvi]

I've added two new options to FormatOptions:

• include_default_or_required_comment (controls whether to include comments about required/default values) and
• comment_out_default_values (controls whether default values are commented out or written as active fields).

Both default to true.

To achieve comment_out_default_values, I've added a new field method to the Formatter trait for emitting a field with its value.

Some Examples:

• Default Options:

toml::template::<Conf>()	yaml::template::<Conf>()	json5::template::<Conf>()
# Port to listen on. # # Can also be specified via environment variable `PORT`. # # Default value: 8080 #port = 8080 # Bind address. # # Default value: "127.0.0.1" #address = "127.0.0.1" [log] # Default value: true #stdout = true #file = # Default value: ["debug"] #ignored_modules = ["debug"]	# Port to listen on. # # Can also be specified via environment variable `PORT`. # # Default value: 8080 #port: 8080 # Bind address. # # Default value: 127.0.0.1 #address: 127.0.0.1 log: # Default value: true #stdout: true #file: # Default value: [debug] #ignored_modules: [debug]	{ // Port to listen on. // // Can also be specified via environment variable `PORT`. // // Default value: 8080 //port: 8080, // Bind address. // // Default value: "127.0.0.1" //address: "127.0.0.1", log: { // Default value: true //stdout: true, //file: , // Default value: ["debug"] //ignored_modules: ["debug"], }, }

• FormatOptions::comment_out_default_values = false

toml::template::<Conf>()	yaml::template::<Conf>()	json5::template::<Conf>()
# Port to listen on. # # Can also be specified via environment variable `PORT`. # # Default value: 8080 port = 8080 # Bind address. # # Default value: "127.0.0.1" address = "127.0.0.1" [log] # Default value: true stdout = true #file = # Default value: ["debug"] ignored_modules = ["debug"]	# Port to listen on. # # Can also be specified via environment variable `PORT`. # # Default value: 8080 port: 8080 # Bind address. # # Default value: 127.0.0.1 address: 127.0.0.1 log: # Default value: true stdout: true #file: # Default value: [debug] ignored_modules: [debug]	{ // Port to listen on. // // Can also be specified via environment variable `PORT`. // // Default value: 8080 port: 8080, // Bind address. // // Default value: "127.0.0.1" address: "127.0.0.1", log: { // Default value: true stdout: true, //file: , // Default value: ["debug"] ignored_modules: ["debug"], }, }

• FormatOptions::include_default_or_required_comment = false

toml::template::<Conf>()	yaml::template::<Conf>()	json5::template::<Conf>()
# Port to listen on. # # Can also be specified via environment variable `PORT`. #port = 8080 # Bind address. #address = "127.0.0.1" [log] #stdout = true #file = #ignored_modules = ["debug"]	# Port to listen on. # # Can also be specified via environment variable `PORT`. #port: 8080 # Bind address. #address: 127.0.0.1 log: #stdout: true #file: #ignored_modules: [debug]	{ // Port to listen on. // // Can also be specified via environment variable `PORT`. //port: 8080, // Bind address. //address: "127.0.0.1", log: { //stdout: true, //file: , //ignored_modules: ["debug"], }, }