CLI: set decryption time on create.

Added an option to set the target decryption time on database creation
for the CLI create command. This required some refactoring, in
particular the extraction of the min, max and defaut decryption times in
the `Kdf` module. Some work was done to allow changing those constant
only in the `Kdf` module, should we ever want to change them.

share/docs/man/keepassxc-cli.1

@@ -1,4 +1,4 @@
-.TH KEEPASSXC-CLI 1 "June 15, 2019"
+.TH KEEPASSXC-CLI 1 "Jan 04, 2020"
 
 .SH NAME
 keepassxc-cli \- command line interface for the \fBKeePassXC\fP password manager.
@@ -179,6 +179,12 @@ Copies the current TOTP instead of current password to clipboard. Will report
 an error if no TOTP is configured for the entry.
 
 
+.SS "Create options"
+
+.IP "\fB-t\fP, \fB--decryption-time\fP <time>"
+Target decryption time in MS for the database.
+
+
 .SS "Show options"
 
 .IP "\fB-a\fP, \fB--attributes\fP <attribute>..."