feat: Add configuration for add_newline (#116)

- Replace TableExt with a Config trait that extends toml::value::Table
Add configuration for add_newline
- add_newline is a root-level configuration value. When set to false, the initial newline before the prompt is removed.

tests/testsuite/common.rs

@@ -9,8 +9,8 @@ lazy_static! {
     static ref EMPTY_CONFIG: PathBuf = MANIFEST_DIR.join("empty_config.toml");
 }
 
-/// Run an instance of starship
-fn run_starship() -> process::Command {
+/// Render the full starship prompt
+pub fn render_prompt() -> process::Command {
     let mut command = process::Command::new("./target/debug/starship");
 
     command
@@ -22,6 +22,7 @@ fn run_starship() -> process::Command {
     command
 }
 
+/// Render a specific starship module by name
 pub fn render_module(module_name: &str) -> process::Command {
     let mut command = process::Command::new("./target/debug/starship");