ASCII UML

There has been a trend in software development to present non-code artifact via plain text, often called ASCII.
Because ASCII has been the most used encoding, and it became part of Unicode,
and UTF-8 has the exactly the same codes.

ASCII usage has been extended for general graphics, often called AsciiArt (see http://asciiflow.com, http://www.jave.de)
and UML, e.g.

PlantUML is the most rich. TextUML is for class diagrams only.
For web tool for sequence diagram
see https://github.com/bramp/js-sequence-diagrams
and http://sequencediagram.org (PlantUML syntax) with additional drag-n-drop of image elements on the right side.

Tooling for PlantUML

Prerequisite. For local preview you need to download/install Graphviz.
And possibly GRAPHVIS_DOT environment variable having full path and name to dot executable.

IDEA

IDEA (Ultimate and CE) has Markdown plugin that can have PlantUML support enabled.
In the Settings/Preferences dialog ⌘,,
select Languages & Frameworks | Markdown.
(https://www.jetbrains.com/help/idea/markdown.html#diagrams)
However that will do only sequence diagram, and for all other types of diagram install “PLantUML integration” plugin.