Add ability to install at generation time

[DannyBen]

Currently, the core generate command usage pattern is

completely generate [CONFIG_PATH OUTPUT_PATH]

If we wish to install at generation time, then there is no point in having OUTPUT_PATH.

My proposal is to change the pattern to

completely generate [CONFIG_PATH OUTPUT]

Where OUTPUT can be:

• a path = same behavior as the live one
• a hyphen = output to stdout, already implemented for 0.7.3
• a command name prefixed by @ - for exampe, @mycli = this will install it for this command

This will allow acrobatics like this:

some_config_generator | completely generate - @mycli

If anyone (@christerswahn) has thoughts or concerns, please comment.

[christerswahn]

Perhaps following standard shell argument conventions would be preferable to an unusual @ syntax, for example:

some_config_generator | completely generate - --install mycli

[DannyBen]

This means there will need to be special handling of cases where both output and --install are provided.

[DannyBen]

I think I can do this:

Usage:
  completely generate [CONFIG_PATH OUTPUT_PATH --function NAME --wrap NAME]
  completely generate [CONFIG_PATH --install PROGRAM --function NAME]
  completely generate (-h|--help)

meaning:

1. --install cannot be used if OUTPUT_PATH is given, and
2. --wrap cannot be used if --install is given

[DannyBen]

Implemented in #90

[DannyBen]

Released in v0.7.3.