Switch from ansi_term to yansi (#288)

ansi_term is not actively maintained anymore (see
https://rustsec.org/advisories/RUSTSEC-2021-0139). Replace it with a
suitable alternative.

I looked at termcolor, owo-colors and yansi and picked yansi:

- It is very simple
- It does not do terminal color support checking (we already do that in
  tealdeer)
- Its color enum type is almost identical to the one from ansi_term, so
  migrating is easy

The only change from a config API point of view is that "purple" is now
renamed to "magenta", but "purple" still works.

Cargo.lock

@@ -17,15 +17,6 @@ dependencies = [
  "memchr",
 ]
 
-[[package]]
-name = "ansi_term"
-version = "0.12.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d52a9bb7ec0cf484c551830a7ce27bd20d67eac647e1befb56b0be4ee39a55d2"
-dependencies = [
- "winapi",
-]
-
 [[package]]
 name = "anyhow"
 version = "1.0.65"
@@ -1111,7 +1102,6 @@ dependencies = [
 name = "tealdeer"
 version = "1.5.0"
 dependencies = [
- "ansi_term",
  "anyhow",
  "app_dirs2",
  "assert_cmd",
@@ -1129,6 +1119,7 @@ dependencies = [
  "tempfile",
  "toml",
  "walkdir",
+ "yansi",
  "zip",
 ]
 
@@ -1553,6 +1544,12 @@ dependencies = [
  "dirs",
 ]
 
+[[package]]
+name = "yansi"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "09041cd90cf85f7f8b2df60c646f853b7f535ce68f85244eb6731cf89fa498ec"
+
 [[package]]
 name = "zip"
 version = "0.6.2"