add support for 'exts_default_options' easyconfig parameter

[boegel]

We currently use something like this in some easyconfig that have a long list of extensions (e.g. R):

ext_options = { ... }

exts_list = [
    ('<name1>', '<version1>', ext_options),
    ...,
]

(where ext_options is just a local variable in the easyconfig file, EasyBuild itself ignores it when parsing)

In places where we need to list additional options, we do ugly things like:

    ('<name2>', '<version2>', dict(ext_options.items() + [('patches', ['example.patch'])])),

This change adds support for exts_default_options, which cleans up this mess thoroughly, as follows:

exts_default_options = { ... }

exts_lists = [
    ('<name1>', '<version1>'),
    ('<name2>', '<version2>', {
        'patches': ['example.patch'],
    }),

This is particularly useful when we want to enforce having checksums for all extensions...

[wpoely86]

add something with a %(...)s too?