add an option to select the application to build an escript for

[galdor]

In the current state, in a multi-application project, the escriptize plugin
will only allow building a single application as an escript, and will force
developers to provide a "main" application with the escript_main_app
configuration variable.

This commit add a command line option for the escriptize plugin which let the
user select the application to build the escript for. The escript_main_app
configuration variable is still used if it is set for backward compatibility.

[galdor]

See https://github.com/galdor/rebar3-test/tree/multi-escripts for an example. rebar3 escriptize -n escript1 and rebar3 escriptize -n escript2 build both escripts separately.

[ferd]

First of all thanks for the contribution. There are a few things to change to get it in line with the way most rebar3 options work, and that is:

• respecting the priority between command-line and config options
• using command-line options that map to those in config files when possible

As such, we should use --main-app (short -a or -m at your leisure) to fit with escript_main_app and/or --name (short -n) to match the escript_name parameters from config.

Right now --name is used for both indirectly, and the command line options should take precedence over the config written in rebar.config, such that if I have a config file specifying {escript_main_app, app1} but call it as rebar3 escriptize --main-app=app2, then I get app2 as the main app we build for.

As the code currently stands, the command line option is a default value for the config, and the naming isn't aligned with the stuff in the config file.

[galdor]

It makes sense, I updated the patch to use --main-app/-a and to change the selection order so that the command line option overrides escript_main_app.

[galdor]

Thank you so much for merging it so quickly !