Create command that will run todotxt-ui on local file(s) and serve on localhost

[bobwhitelock]

E.g. like $ todotxt-ui todo.txt.

Should be able to specify config file (from #122). Possibly also specify config params via options, e.g. --kanban for #123.

Running Git commands should maybe be optional in this mode, as the use case I can think of is rapid local management without needing to make many small commits, or trying out/running the app without having a Git repo set up.

Maybe this should even be the main way to run the app (related to #95?