Allow completion_number to take a configurable value.

As per https://www.pivotaltracker.com/s/projects/911874/stories/65892596

No new tests, since the parent completion collection handles all the logic,
and this is already tested.
Fix JSLint errors
Let completion graphs have flexible axis period.

* Allow completion collection and completion_numbers module to have axis
  period.
* Update completion graph and submission graph to use axis period where
  avialable.
* Create tests for completion graph and submission graph, ensure that
  configNames use collection period where available.

Part of this story: https://www.pivotaltracker.com/s/projects/911874/stories/65892596

Builds on Edd's work in https://github.com/alphagov/spotlight/pull/192
Ensure completion modules pass duration option.

Also add axisPeriod option to completion_rate module, since this wasn't
previously specified.
Update labelPrefix to include correct period.

Not sure if we need to add a new test for this file, since this is so minor?
Fix JSLint errors
Improve naming of startMatcher/endMatcher.

The use of 'startMatcher' and 'endMatcher' is technical debt from when
the completion collections were only used to calculate the percentage
of users ending a transaction, compared to the number of users starting
the transaction.

Now we want to use them to calculate more generic percentages - e.g.
the percentage of transactions that are digital. So it is confusing to
use 'start' and 'end' when what we really mean is 'denominator' and
'numerator'.

Rename accordingly.

Part of this story: https://www.pivotaltracker.com/s/projects/911874/stories/63460908
Rename start-matcher and end-matcher.

As part of our wider change to make the naming of completion modules more
intuitive, use "numerator" and "denominator" rather than "end" and "start"
throughout the project.
Merge pull request #228 from alphagov/feature/flexible-filled-line-graph

Feature/flexible filled line graph