Closes #104

Why

Some users prefer not to see shell tracing output from the xtrace option. This is more of a user preference than a requirement based on the task in question, so it makes sense to me to offer an environment variable that users can set for a specific command invocation or put in their .env to disable tracing for all commands if that's their preference.

What

• Adds a new member to the Runner struct, trace
• Sets trace based on the value of the XC_TRACE env variable upon runner instantiation. Specifically, a value of "false", "no", or "0" (or any uppercase variant thereof) disables tracing by setting trace = false.
• Respects the trace preference by writing set -o xtrace only when not disabled.
• Documents the env variable