Bugfix: correct malformed HTML code in versioned docs redirect

HTML is harder than it looks when you haven't done it in a while.

Multiversioned Doxygen Docs

Also:

• improved documentation
• replaced project placeholder name with one that is easier to find-and-replace
• mothballed Travis-CI config
• reduced MSVC warning level from W4 to W3

Modern CMake!

• multi-directory source code organisation, separate for headers etc...
• use CPM to grab dependencies such as Catch, for easier testing on CI
• demonstration of where to put public and private header files
• Usage of psuedo-targets in CMake to propagate project-wide options, rather than setting them globally
• CMake project is exported properly such that it can be found automatically by other CMake projects once installed (install(EXPORT))

Separate CI / Release builds

• CI is only run for pushes to master and for any pull request
• Release builds are only made for releases (also, produce only one release per OS and simplify file names)

Tidy up

v0.1.2

Remove mistakenly-added directory

HOTFIX: Release build detection

Whoops! Use github.event_name rather than github.event

This was a mistake in the release-checking logic

Added CI on Github Actions

Added config file to support Continuous Integration using Github Actions:

• Same cross-platform building as on Travis-CI
• Additionally, supports building releases automatically on each platform and storing these for convenience (Github Action build Artifacts)