jdblischak/workflowr: workflowr 1.4.0

This minor release of workflowr features further GitHub integration, a new reproducibility check, and various improvements and bug fixes.

• The initial GitHub setup always included the manual step of creating the GitHub repository, but this is no longer the case! When you run wflow_use_github("username"), it will offer to create the new GitHub repository for you. If you sign-in to GitHub via your web browser and grant workflowr permission, it will be created automatically.

• Absolute paths to files on your local computer are not reproducible. If you or someone else tries to execute the code on a different machine, it will fail. Now workflowr will automatically search for absolute paths to files that are inside of the workflowr project. If it detects any, it will fail the reproducibility check, and provide you with the equivalent relative paths to use.

• Now every time you push your latest changes with wflow_git_push(), a new browser tab will automatically be opened to your online repository (idea from @pcarbo)

• Mention knitr::include_graphics() as an option for including external images (idea from @Zepeng-Mu, #162)
• Check if Git repository is locked and produce error message that explains how to fix it (idea from @brimittleman)
• Document that wflow_build() and wflow_publish() build the files in the given order if they are provided explicitly to the argument files (idea from @antass and @pcarbo, #164)
• Handle spaces in chunk names for HTML targets. If a chunk name contains a space, the name of the figure file will also has a space. This broke the accompanying link for the HTML button with the table of previous versions of the figure (because it uses the filename to be unique). Now the spaces are removed in the HTML link.
• Add FAQ entry on installing packages in a workflowr project (idea from @xiangzhu, #160)
• wflow_use_github()/wflow_use_gitlab() provide better guesses if the arguments username or repository are left blank. If repository is NULL, it is set to the name of the workflowr project directory (idea from @pcarbo).
• wflow_git_push() and wflow_git_pull() no longer accept direct URLs to remote repositories. The argument remote must be NULL or the name of an existing remote. The support for direct URLs was likely rarely used since it is rarely used with Git as well, and it likely never worked given how the underlying functions from git2r work.
• Document in the GitLab vignette that the repository will be automatically created the first time the repository is pushed with wflow_git_push() (this is a feature unique to GitLab)