Added an option for ignoring errors about missing modules

[tumik]

This commit adds an option (-i) to ignore errors about missing modules.

Ignoring missing modules is really useful for using iverilog as linter in text editors (like Atom or Sublime Text), where you usually only want to check single files without caring about modules on other files.

[steveicarus]

This looks fine and useful.

I am curious what happens when you use this flag in a non-linting situation.

[jersey99]

Does seem really useful, and I gave this a try. However, if the missing file is included with an `include directive, ivlpp still errors out:

Include file xxx.vh not found

Not sure what the intended behavior in this case should be.

[jfcherng]

@martinwhitaker I wonder can we have this PR included in the v10-branch as well? Currently it seems to be only in master. I just found that the iverilog package on Ubuntu does not have the -i flag which is quite useful for linter usage...

[martinwhitaker]

We don't normally add new features to the stable branch, but this seems safe enough.
@steveicarus , what do you think?

[jfcherng]

@martinwhitaker @steveicarus sorry to re-ping on this but any chance to land it on v10?