Partial Revert of spf13#922 (spf13#1068)

Issue Reference: spf13#1056

spf13#922 introduced a new error
type that emitted when a command was not runnable. This caused
all commands w/o a run function set to error w/ that message and a status code of 1.

This change reverts the addition of that new error. Similar
functionality can be accomplished by leveraging RunE.

Partial Revert of spf13#922 (spf13#1068)

Issue Reference: spf13#1056

spf13#922 introduced a new error
type that emitted when a command was not runnable. This caused
all commands w/o a run function set to error w/ that message and a status code of 1.

This change reverts the addition of that new error. Similar
functionality can be accomplished by leveraging RunE.

Add support for context.Context

fixing linter issues

fixing linter issues

added variable to allow configuration of mousetrap message duration (s…

…pf13#809)

new variable MousetrapDisplayDuration allows to modify the default
display duration of 5s, or to completely disable the timeout and wait
for the user to press the return key.

Fixed code sample for bash completion (spf13#687)

Update the Travis and CircleCI Go versions (spf13#651)

Create new buffer if not present yet (spf13#549)

Fixes a nil dereference when TraverseChildren is used
with multiple subcommands.