v0.4.0

Fixes:

• fixed parse_file() failing to compile with plain string literals
• fixed tests being built when used as a meson subproject (#14) (@shdnx)

Additions:

• added support for compiling into DLLs on windows (TOML_API)
• added support for explicitly setting the TOML_EXCEPTION mode
• added TOML_OPTIONAL_TYPE customization point
• added node::ref() and node_view::ref()