fix: handle path to shell in starship init (#106)

Add support for paths to init function

This adds support for qualified paths (e.g. using
`/usr/local/bin/zsh` instead of `zsh`) to init.rs.

The function now converts the shell name into an
OsStr, then to a Path, then gets the file stem,
and unwraps back into a str. While this process can
fail (yielding a None), it's highly unlikely to unless
the user has messed with their shells or there's an
issue in Starship--therefore, the failure message in
this case simply asks the user to file a bug report.

src/init.rs

@@ -1,25 +1,43 @@
+use std::ffi::OsStr;
+use std::path::Path;
+
 pub fn init(shell_name: &str) {
     log::debug!("Shell name: {}", shell_name);
-    let setup_script = match shell_name {
-        "bash" => {
+
+    let shell_basename = Path::new(shell_name).file_stem().and_then(OsStr::to_str);
+
+    let setup_script = match shell_basename {
+        Some("bash") => {
             let script = "PS1=\"$(starship prompt --status=$?)\"";
             Some(script)
         }
-        "zsh" => {
+        Some("zsh") => {
             let script = "PROMPT=\"$(starship prompt --status=$?)\"";
             Some(script)
         }
-        "fish" => {
+        Some("fish") => {
             let script = "function fish_prompt; starship prompt --status=$status; end";
             Some(script)
         }
+        None => {
+            println!(
+                "Invalid shell name provided: {}\\n\
+                 If this issue persists, please open an \
+                 issue in the starship repo: \\n\
+                 https://github.com/starship/starship/issues/new\\n\"",
+                shell_name
+            );
+            None
+        }
         _ => {
+            /* Calling unwrap() here is fine because the None case will have
+            already matched on the previous arm */
             println!(
                 "printf \"\\n{0} is not yet supported by starship.\\n\
                  For the time being, we support bash, zsh, and fish.\\n\
                  Please open an issue in the starship repo if you would like to \
                  see support for {0}:\\nhttps://github.com/starship/starship/issues/new\"\\n\\n",
-                shell_name
+                shell_basename.unwrap()
             );
             None
         }