Add plotting support for text-based intervals

[zkamvar]

In #43, stable datetime support was added, which meant that strings such as "day", "week", "month", etc could be used to define intervals. This PR adds plotting support for these strings and allows for variable width intervals in the plot at least.

Things fixed/added:

• support for string-based interval added
• new internal functions
  • floor_months() sets the date to the first day of the given month
  • add_month() takes a vector of three characters YYYY, MM, and DD, and adds a number of months, returning the "YYYY-MM-DD" string.
  • get_days_in_*() takes a vector of dates and returns a vector of days in a (month|quarter|year) to the right of each date.
• support for variable intervals for each count

Things that may still need to be done, but don't necessarily have to be done for this PR to be pulled:

• Shifting bins to the beginning/end of the month in the incidence constructor
• Validating "quarter" plotting

Example

library("incidence")
library("outbreaks")
d <- ebola_sim_clean$linelist$date_of_onset
plot(incidence(d, interval = "day"))

plot(incidence(d, interval = "week"))

plot(incidence(d, interval = "month"))

plot(incidence(d, interval = "quarter"))

Created on 2018-07-12 by the reprex package (v0.2.0).

[thibautjombart]

I think the travis failing is an encoding issue in the doc - can you check? See log below.

* checking files in ‘vignettes’ ... OK
* checking examples ... OK
* checking for unstated dependencies in ‘tests’ ... OK
* checking tests ...
  Running ‘testthat.R’
 OK
* checking for unstated dependencies in vignettes ... OK
* checking package vignettes in ‘inst/doc’ ... OK
* checking re-building of vignette outputs ... OK
* DONE
Status: 1 WARNING
See
  ‘/home/travis/build/reconhub/incidence/incidence.Rcheck/00check.log’
for details.
0.46schecking Rd files ... WARNING
estimate_peak.Rd: non-ASCII input and no declared encoding
problem found in ‘estimate_peak.Rd’