Session config

[joshmedeski]

Closes #86

• Introduces [[session]] configuration options for creating custom sessions with startup functionality
• [[session]] are listed in session list -c using the name property of each session configuration
• [[session]] requires a path option to define the base directory for that session
• [[session]] supports startup_script or startup_command options that is ran when the custom session is connected to

Breaking Changes

Dropping [[startup_scripts]] in place of [[session]]

-[[startup_scripts]]
-session_path = "~/c/joshmedeski.com"
-script_path = "~/. config/sesh/scripts/node_dev"
+[[session]]
+name = "joshmedeski.com"
+path = "~/c/joshmedeski.com"
+startup_script = "~/. config/sesh/scripts/node_dev"

Dropping default_startup_script in place [default_session] which supports startup_script or startup_command options

-default_startup_script = "~/.config/sesh/scripts/open_files"
+[default_session]
+startup_script = "~/.config/sesh/scripts/open_files"