feat: set the program environment variables

This gives the option to provide a custom set of environment variables
to use in the Bubble Tea program. When running Bubble Tea in a remote
session such as SSH, we need to be able to pass the SSH session's
environment variables rather than the server ones.

options.go

@@ -49,6 +49,13 @@ func WithInputTTY() ProgramOption {
 	}
 }
 
+// WithEnvironment sets the environment variables that the program will use.
+func WithEnvironment(env []string) ProgramOption {
+	return func(p *Program) {
+		p.environ = env
+	}
+}
+
 // WithoutSignalHandler disables the signal handler that Bubble Tea sets up for
 // Programs. This is useful if you want to handle signals yourself.
 func WithoutSignalHandler() ProgramOption {

tea.go

@@ -151,6 +151,9 @@ type Program struct {
 	previousOutputState *term.State
 	renderer            renderer
 
+	// the environment variables for the program, defaults to os.Environ().
+	environ []string
+
 	// where to read inputs from, this will usually be os.Stdin.
 	input io.Reader
 	// ttyInput is null if input is not a TTY.
@@ -222,6 +225,11 @@ func NewProgram(model Model, opts ...ProgramOption) *Program {
 		p.output = os.Stdout
 	}
 
+	// if no environment was set, set it to os.Environ()
+	if p.environ == nil {
+		p.environ = os.Environ()
+	}
+
 	return p
 }