Added optional export of private key

[scx1332]

For some implementations it's useful feature, when you store your keys in keystore, but also you want possibility of getting plain private key.

[scx1332]

I've found that you can hack your key using encrypt and decrypt which returns Vec:

let raw_key = secret.to_crypto(password, 1).unwrap().decrypt(password).unwrap();

Still that solution that I posted is more elegant.

[tomusdrw]

Looks good! I'd appreciate if you could fix minor readme changes and comment on calling the method raw instead of private.

We could also change the feature name to expose-raw-private-key or sth like that?

[tomusdrw]

Suggested change

	but you can add --features export-private-key to export it.
	but you can enable `export-private-key` feature to export it.

[tomusdrw]

Suggested change

	Library by defaults hide private key from access,
	Library by default hides raw private key from being accessed,

[tomusdrw]

Suggested change

	//Do not print private key in that way in production code
	// Do not print private key in that way in production code!

[tomusdrw]

Suggested change

	//Do not print private key in that way in production code
	// Do not print private key in that way in production code!

[tomusdrw]

Suggested change

	/// Export stored, unencrypted, plain private key, use with caution
	/// Do not expose this key in logs, etc. Use only if needed
	/// Export stored, unencrypted, plain private key.
	///
	/// USE WITH CAUTION! Do not expose this key in logs,
	/// etc. Use only if really needed.

[tomusdrw]

Suggested change

	pub fn private(&self) -> [u8; 32] {
	pub fn raw(&self) -> [u8; 32] {

WDYT about calling the function raw instead of private? I think it makes sense as it returns the raw key.