Support a `eventual tail execution` command.

[thantos]

@amildahl pointed out that tail traditionally returns the end of a file, while -f or --follow is what continues to read the end of the file. (or shift-F in less).

Changes

eventual start workflow <workflow> [--follow|-f] # start the workflow, if `-f` is given, return all new events as they occur
eventual tail execution <execution> [-n] [--follow|-f] # works on a running execution, returns the last N events, when -f is given, watch the workflow

[sam-goodwin]

Makes sense. The Unix command is tail -f