Add support for termcolor 1.2

Widen list of supported termcolor versions.  If the corresponding
cargo feature is enabled, termcolor can now be at any of the
versions 1.0.*, 1.1.* or 1.2.*.

Cargo.toml

@@ -21,7 +21,7 @@ maintenance = { status = "actively-developed" }
 [dependencies]
 ansi_term = { version = "0.12", optional = true }
 rgb = { version = "0.8", optional = true }
-termcolor = { version = "1.1", optional = true }
+termcolor = { version = ">= 1.0, <= 1.2", optional = true }
 
 [features]
 default = ["rgb"]